听力原文：Professor Well, today wed talk about Force and its representation. Student A S

o what is force？ Professor Well, actually, a force is a push or pull upon an object resulting from the objects interaction with another object. Whenever there is an interaction between two objects, there is a force upon each of the objects. When the interaction ceases, the two objects no longer experience the force. Forces only exist as a result of an interaction. Student A interrupting You mean if a force exists, there should be two objects. Am I right？ Professor Thats right. Er, ...For simplicity sake, all forces, interactions, between objects can be placed into two broad categories. One is contact force; the other is force resulting from action-at-a-distance. Student B interrupting Action-at-a-distance？ Professor Yeah, first lets talk about contact force. Contact forces are types of forces in which the two interacting objects are physically in contact with each other. Examples of contact forces include frictional forces, tensional forces, normal forces, air resistance forces, and applied forces. These specific forces will be discussed in more detail later in Lesson 2, as well as in other lessons. Student A Uh...so many forces fall into this category. Then, what is "force resulting from action-at-a-distance"？ Professor Er Action-at-a-distance forces are types of forces in which the two interacting objects are not in physical contact with each other, yet are able to exert a push or pull despite a physical separation. Examples of action-at-a-distance forces include gravitational forces, such as, the sun and planets exert a gravitational pull on each other despite their large spatial separation; even when your feet leave the earth and you are no longer in contact with the earth, there is a gravitational pull between you and the Earth. Next, electric forces, such as, the protons in the nucleus of an atom and the electrons outside the nucleus experience an electrical pull towards each other despite their small spatial separation; and magnetic forces, for example, two magnets can exert a magnetic pull on each other even when separated by a distance of a few centimeters. These specific forces will be discussed in more detail in other lessons. Student B Thats magic! I am quite familiar with the phenomenon you mentioned just now, but before this class, I had no concept of these categories of forces. Student A Er, I know very little about this, too. Professor Actually, forces exist in our daily life. Force is a quantity which is measured using the standard metric unit known as the Newton. A Newton is abbreviated by an "N". To say "10.0 N" means 10.0 Newtons of force. Student A interrupting So Newton is the standard metric unit of force？ Professor Thats true. In fact, a force is a vector quantity. As we learned in an earlier unit, a vector quantity is a quantity which has both magnitude and direction. To fully describe the force acting upon an object, you must describe both the magnitude and the direction. Thus, "10 Newtons" is not a full description of the force acting upon an object. In contrast, "10 Newtons downwards" is a complete description of the force acting upon an object; both the magnitude, "10 Newtons", and the direction, "downwards", are given. Student B Uh, thats quite complex and easy to make a mistake. Professor Yeah because a force is a vector which has a direction, it is common to represent forces using diagrams in which a force is represented by an arrow. Such vector diagrams were introduced in an earlier unit and will be used throughout your study of physics. The size of the arrow is reflective of the magnitude of the force and the direction of the arrow reveals the direction which the force is acting. Such diagrams are known as free-body diagrams. Furthermore, because forces are vectors, the influence of an individual force upon an object is often canceled by the influence of another force. For example, the influence of a "20 Newton upward" force acting upon a book is canceled by the influence of a "20 Newton downward" force acting upon the book. In such instances, it is said that the two individual forces "balance each other", meaning there would be no unbalanced force acting upon the book. Student A That sounds interesting. Then, there should be other situations in which two of the individual vector forces cancel each other, the so-called balance, yet a third individual force exists that is not balanced by another force. Professor Thats the point. Next time we will learn how to draw the free-body diagrams, which can help you to understand the situation more easily.Narrator Listen to part of a discussion in a physics class. The professor is discussing Forces. Now get ready to answer the questions. You may use your notes to help you answer.

What is the discussion mainly about？

A．The characteristics of forces.

B．The name of various forces.

C．The two categories of forces.

D．The diagrams of forces.