According to this passage, the Roman soldiers mainly depended upon ______to send their messages.

Greek soldiers sent messages by turning their shields toward the sun. The flashes 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.