Children are curious about the world around them. For example, they want to know【36】their

hearts beat. They want to know why the ocean water【37】salty.

【38】children grow up, they become curious about different kinds of things. When they are babies, they are interested in the parts of their bodies and in the smiles of their mothers.【39】they become interested in the physical world around them the plants, the animals, the sky. Later, they become interested in the things【40】people have made: wheels, bicycles, cars. And when they are adults, their curiosity【41】. Sometimes this curiosity leads to a【42】in science.

Scientists spend their lives【43】to find out about the world.【44】who work with the earth sciences study the earth, the oceans, and the skies.【45】scientists who study living work with the biological sciences. A third group of scientists study the physical sciences, e.g. physics, chemistry.

(36)

A．what

B．how

C．which

D．when