A．In 236 BC. B．In the Middle Ages. C．During World War I. D．During the Industrial Revolution.
A．Wind. B．Gas. C．Steam power. D．Solar power.
A．The US. B．Italy. C．China. D．Russia.
A．How to safeguard the computer network. B．How to steal top secret files from a military base. C．How to make modern devices broadcast invisible, inaudible signals. D．How to use an FM radio to detect the invisible, inaudible signals.