题目内容：

Scientists in Colombia have trained a team of rats to help remove land mines from the countryside. Land mines are small bombs hidden in the ground and designed to explode(爆炸) when someone steps on them.

When trained, the rats can smell special metals in the mines and then signal(发信号) to their human team. Then why use rats for such an important job?

In the past, dogs have been used to find bombs. Rats have a good sense of smell and can respond to orders, just like dogs do. But rats are much lighter than dogs. They stand less of a chance of setting off the mines, which explode under a certain amount of weight. What’s more, the best rats are cheaper to keep than dogs. It costs about the same amount of money to take care of one dog as it does 70 rats.

The rats have already been used in Tanzania, an African country with many land mines. Luisa Fernanda Méndez and her team in Colombia are training the even lighter Wistar rats. These are the white rats with red eyes that scientists often use in laboratory experiments. “They even train their babies to perform their jobs, which saves us a lot of time, ” Méndez said.

Colombia has the second-highest rate(比率) of deaths from land mine accidents in the world. Last year, land mines killed 695 people — 56 of whom were children. A criminal group called the Revolutionary Armed Forces of Colombia, or FARC, as well as other organizations, have placed the mines. FARC are strongly against Colombia’s government. The group has placed mines all around its own bases and in many places in the countryside.

In the past, the Colombian government hired peasants and poor farmers to find mines. Many of them died trying to find the well-hidden bombs.

Colombian police official Erick Guzmán hopes the team of rats can come to the rescue(援救). “These rats will be a great help,” he said.

1.How do rats help to find land mines?

A．By their special sight.                     B．By their light weight.

C．By their sense of smell.                   D．By their excellent hearing.

2.The advantage of rats over dogs in finding bombs is that _____.

A．they are cleverer.

B．they run faster

C．they have a better sense of smell

D．they don’t make mines explode as easily

3.The underlined word “They” may refer to ______.

A．scientists                              B．Wistar rats

C．dogs                                 D．Méndez and her team

4. What is the passage mainly about?

A．Specially-trained rats help clear land mines.

B．People in Tanzania are suffering from land mines.

C．Dogs are no longer used to find land mines.

D．Land mines have become a great danger to Colombians.