Some unfortunate people are rather attractive to mosquitoes, while the smell of some lucky people drives the blood-suckers away. Now the smelly chemicals from the sweat of these lucky people have been identified by researchers, who are testing its effectiveness as a natural mosquito repellent (驱蚊剂).

Everybody produces a mixture of chemicals with smells in their sweat, some of which attract biting insects. But people who do not get bitten also produce smelly chemicals that appear to mask the smells of the attractive chemicals.

This masking is very effective in avoiding a mosquito’s bite, explains James Logan, a scientist who carried out the research with his colleagues.

These helpful chemicals probably occur naturally in everybody’s sweat but some people have a much higher ratio (比例) than others.

“It’s very exciting,” Logan says, “because these are totally natural chemicals with an effectiveness that compares favorably to other chemicals such as DEET, which is the best repellent available but has unwanted side effects.”

A key chemical identified by Logan as a repellent is also “a natural food additive which has been proven safe”. He says, “And because it can be made by plants, it may one day be possible to mass-produce it cheaply.”

The repellent is currently being tested by 16 volunteers in Africa. Meanwhile, the researchers are testing the repellent against other biting insects.

49．Which can be concluded from this passage?

     A．Mosquitoes like to suck attractive people.

     B．Only some people produce a smelly substance in their sweat

     C．The cost of plant-related products can be low.

     D．Some chemicals contained in people’s sweat can be eaten as food.

50．What is the main idea of this passage?

     A．Some people are luckier than other people

     B．Scientists help us to avoid the inconvenience of the biting insects

     C．Scientists have found a natural anti-biting substance in people’s bodies.

     D．The natural mosquito repellent is good for people.

51．Why are some people less attractive to mosquitoes?

     A．Because they have less sweat    

     B．Because their sweat contains a particular chemical.

     C．Because the ratio of their natural mosquito repellent is much higher

     D．Because they are less attractive

52．Which of the following statements is NOT true according to this passage?

     A．Scientists are doing experiments with volunteers in Africa.

     B．The natural mosquito repellent is only used to deal with mosquitoes

     C．DEET is the best repellent available now

     D．All people might produce chemicals that can mask the mosquito-attracting substance.