听力原文: Greek soldiers sent messages by turning their shields toward the sun. The flashe
[16]In Africa, people learned to send messages by beating on a series of large drums. Each drum was kept within hearing distance of the next one. The drum beats were sent out in a special way that all the drummers understood. Though the messages were simple, they could be sent at great speed for hundreds of miles.
In the eighteenth century, [15]a French engineer found a new way to send short messages. In this way, a person held a flag in each hand and the arms were moved to various positions representing different letters of the alphabet. It was like spelling out words with flags and arms.
Over a long period of time, people sent messages by all these different ways. [17]However, not until the telephone was invented in America in the nineteenth century could people send speech sounds over a great distance in just a few seconds.
According to this passage, the Roman soldiers mainly depended upon ______to send their messages.
A.fine weather
B.signal towers
C.the spelling system
D.arm movements