"The noise made by boats of whale-watching tourists may be causing communication problems for the animals," British researchers said last week.
Whale watching is a popular tourist activity in places where groups of killer whales (虎鲸) live. Every day, dozens of boats filled with tourists go to watch the beautiful animal. Unfortunately, the engines of the boats make a lot of noise, and that may be causing problems for the whales, according to a report in the US science magazine Nature .
The whales travel in groups, and communicate with each other through sonic  (声波的)  calls  that some scientists refer to as singing. The sounds of the boat engines may be creating too much background noise for the whales to be able to hear each other.
The researchers listened to recordings of whale calls made between 15 and 25 years ago, before whale-watching became popular. These were compared to more recent recordings.
They found that the animals have started to sing for longer than they used to, like a person shouting to a friend across a noisy room.
It is believed that the whales are trying to communicate hunting or  breeding(繁殖) information. Scientists said that, "if this is prevented, the future of the world's whale population could be in danger."
"Animals became slow in mind and repeat themselves in noisy areas," said Volker Deecke, a Canadian biologist. "They have to say things twice or three times in order to be understood.”
The killer whales studied were living off the coast of northwestern America. The number of killer whales in this area has been falling for several years, but scientists are not sure why.  
The passage mainly tells us______.

A．The problems of whale-watching

B．how killer whales communicate with each other

C．the reduced population of killer whales caused by whale-watching

D．hearing problems of the killer whales caused by whale watching.

Now in a noisy background, the killer whales have to__________.

A．sing in a louder voice

B．sing longer

C．travel in smaller groups

D．get nearer to each other

Scientists believe that killer whales sing to_______.

A．warn each other of dangers

B．keep in the same group

C．send useful information

D．enjoy themselves

From the passage, we can feel that the author is_______.

A．angry

B．hopeful

C．worried

D．disappointed

Which of the following is the possible result of whale-watching?

A．Fewer fish in the ocean.

B．Misunderstanding between the whales.

C．Reduced number of the whales in the area.

D．Serious water pollution along the coast.