题目内容：

A bite from a tsetse fly is an extremely unpleasant experience.To make matters worse,several species of tsetse fly can transmit diseases.One of the most dangerous is a parasite（寄生虫）that causes "sleeping sickness".

After the initial bite,sleeping sickness symptoms often start with a fever,headaches and aching muscles.As the illness goes on,those infected become increasingly tired,which is where it gets its name.

It is worth noting that sleeping sickness is no longer as deadly as it once was.In the early 20th Century several hundred thousand people were infected each year.By the 1960s the disease was considered "under control" and had reached very low numbers,making its spread more difficult.But in the 1970s there was another major infection,which took 20 years to control.Since then,better screening programs and earlier treatments have reduced the number of cases dramatically.In 20is this figure dropped to fewer than 3,000.The World Health Organization (WHO) hopes the disease will be completely removed by 2020.More problematically,a series of new studies have shown that the parasite is more complicated than previously believed.

Sleeping sickness has always been considered—and analyzed—as a blood disease,because the parasites can readily be discovered in the blood of its victims.However,in a recent study found that the parasite can stay in the skin and fat,as well as in the blood.There may even be a higher density（密度）of the parasite in the skin than in the blood. That means a person can have no symptoms but still both harbor the disease and spread it.The finding could explain the mysterious 1970s infection,and why the disease can spring up in areas that had previously been cleared.

1.What's the danger of a tsetse fly bite?

A. It makes people sleepy.    B. It causes skin disease.

C. It brings about deaths.    D. It transmits deadly parasites.

2.What can we infer about sleeping sickness from Paragraph 3?

A. It's still a threat to human health.    B. It's not that dangerous at present.

C. It's incurable in the early 20th century.    D. It's completely under control in the 1960s.

3.What does the underlined "harbor" most probably mean?

A. carry    B. resist    C. exchange    D. hide

4.What does the latest research indicate according to the passage?

A. Sleeping sickness is a blood disease.    B. Skin is more suitable for the parasite to grow.

C. Sleeping sickness can be spread with no signs.    D. Parasites in the skin caused the 1970s infection.