A．an effort to protect an endangered marine species
B．the civilian use of a military detection system
C．the exposure of a U. S. Navy top-secret weapon
D．a new way to look into die behavior of blue whales

A．the sophisticated technology of focusing sounds under water
B．the capability of sound to travel at high speed
C．the unique property of layers of ocean water in transmitting sound
D．low-frequency sounds traveling across different layers of water