Some scientists say that animals in the oceans are increasingly threatened by noise pollution caused by human beings.
The noise that affects sea creatures comes from a number of human activities. It is caused mainly by industrial underwater explosions, ocean drilling, and ship engines. Such noises are added to natural sounds. These sounds include the breaking of ice fields, underwater earthquakes, and sounds made by animals themselves.
Decibels (分贝) measured in water are different from those measured on land. A noise of one hundred and twenty decibels on land causes pain to human ears. In water, a decibel level of one hundred and ninety－five would have the same effect.
Some scientists have suggested setting a noise limit of one hundred and twenty decibels in the oceans. They have observed that noises at that level can frighten and confuse whales(鲸鱼).
A team of American and Canadian scientists discovered that louder noises can seriously injure some animals.
The research team found that powerful underwater explosions were causing whales in the area to lose their hearing. This seriously affected the whales' ability to exchange information and find their way. Some of the whales even died. The explosions had caused their ears to bleed and become infected.
Many researchers whose work depends on ocean sounds are against a limit of one hundred and twenty decibels. They say such a limit would mean an end to important industrial and scientific research.
Scientists do not know how much and what kinds of noises are harmful to ocean animals. However, many scientists don't think that noise is a greater danger than they believed. They want to prevent noises from harming creatures in the ocean.
21．According to the passage, natural sounds include all of the following EXCEPT________.
A．sounds made by animals themselves
B．ocean drilling
C．underwater earthquakes
D．the breaking of ice fields
22．Which of the following is true of whales？
A．They won't be confused by noises.
B．They are deaf to noises.
C．Their ability to reproduce will be lowered by highlevel noises.
D．Their hearing will be damaged by highlevel noises.
23．According to the passage, what will scientists most probably do in the future?
A．They will work hard to reduce ocean noise pollution.
B．They will protect sea animals from harmful noises.
C．They will try to set a limit of 120 decibels.
D．They will study the effect of ocean noise pollution.