题目内容：

Researchers at San Diego Zoo have been studying what has been described as the “secret language” of elephants. They have been monitoring（监测） communications between animals that cannot be heard by human ears.

The elephant’s call will be familiar to most people, but the animals also give out growls (低吼). Their growls, however, are only partly audible (听得见的); two-thirds of the call is at frequencies that are too low to be picked up by our hearing. To learn more about the inaudible part of the growl, the team attached (附在……上面) a microphone sensitive to these low frequencies and a GPS tracking system to eight of the zoo’s female elephants. The researchers could then relate the noises the animals were making to what they were doing. Matt Anderson, who led the project, told BBC News, “We’re excited to learn how they interact and contact with one another.”

The team has already learned that pregnant females use this low frequency communication to announce to the rest of their long gestation (妊娠期) of over two years, in the last 12 days we see the low part of the growl, which we can’t hear. This we believe is to announce to the rest of the herd that the baby is upcoming,” said Dr Anderson.

The researchers believe that this also warns the elephants to look out for coming danger. “You may think that a baby calf of about 300 pounds would not be as open to predation (捕食) as other species,” he says. “But packs of hyenas (袋狼) are a big threat in the wild.”

Female elephants are only in season for around four days every four years and these calls can be heard by males more than two miles away.

1.Why some elephants’ call is called “secret language”?

A．Because it can’t be heard by human ears.

B．Because it can only be heard by female elephants.

C．Because people don’t know its meanings.

D．Because people haven’t studied it completely.

2.By using a sensitive microphone and a GPS tracking system, the researchers wanted to find out             .

A．where the elephants usually go

B．what the elephants’ growls really mean

C．how mother elephants raise baby calves

D．how elephants protect their babies.

3.A pregnant elephant uses inaudible growls to           .

A．show the location where she is staying

B．ask for help when she losses her way

C．scare away the enemy

D．let others know she will have a baby

4.What is the passage mainly about?

A．The elephants can speak like people.

B．The wild elephants’ ways to fight against enemies.

C．The “secret language” between elephants.

D．The special life of female elephants.