听力原文： Greek soldiers sent messages by turning their shields toward the sun. The flashe

s reflected light could be seen several miles away. The enemy did not know what the flashes meant, but other Greek soldiers could understand the messages. [14]Roman soldiers in some places built long rows of signal towers. When they had a message to send, the soldiers shouted it from tower to tower. If there were enough towers and enough soldiers with loud voices, important news could be sent quickly over distance.

[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