Most people do not think of fishes and other sea animals as having voices, and of those who are aware of the fact that many of them can "speak", few understand that these "conversations" have significance. Actually, their talk may be as meaningful as much of our own. For example, some sea animals use their "voices" to locate their food in the ocean expanses (广阔的区域); others use their "voices" to let their fellows know of their locations; and still others, as a means of obtaining mates. Sometimes, "speaking" may even mean the difference between life and death to a sea animal. It appears in some cases that when a predator (食肉动物) approaches, the prey (被捕的动物) depends on no more than the sounds it makes to escape.Fish sounds are important to man, also. By listening to them, he can learn a great deal about the habits of creatures that make them, the size of the School they form, the patterns of their migrations, and the nature of the environments in which they live. He can also apply this information to the more effective utilization of the listening spots he has set up to detect enemy submarines (潜水艇). A knowledge of fish sounds can avoid confusion and unneeded effort when a "new" sound is picked up and the sound sentry (哨兵) must decide whether or not to call an alarm. The last sentence means that a knowledge of fish sounds can help man to （）

A. avoid confusion in fishing activity
B. avoid unneeded effort in fish sound studies
C. detect enemy submarines
D. pick up new sounds