The way of communication ______ made use of were visible signs.

A．in a special way
B．over a very short distance
C．by a musical instrument
D．at a rather slow speed

A．Neither the Greek soldiers nor their enemy could understand the message.
B．African soldiers shouted from tower to tower to pass message.
C．Telephone was invented by a French engineer.
D．Only by using telephone could people send speech sounds quickly.