题目内容：

Sharks use their good sense of smell to search for prey（猎物）. But a new study shows that the rising levels of carbon dioxide in the ocean could reduce sharks' ability to find food. It's bad news for a healthy ocean. Sharks are top predators（掠食动物）and are like the ocean's weed controllers.

Dixson works at the Woods Hole Oceanographic Institution in Massachusetts. Her earlier work has shown that clown fish in acidifying（酸化的）water act strangely. "We want to expand this research to sharks .because we know how important it is for sharks to find their food through their sense of smell," said Dixson.

Her team started by placing sharks in each of three different pools for five days. Each pool had a different level of carbon dioxide in it. The first had the same amount as today's ocean water. The second had a level of carbon dioxide that scientists expect to reach by 2050. The third pool had the highest level of carbon dioxide that could develop by the year 2100.

Sharks weren't allowed to eat any food while in the pools. This made sure that they would be hungry. Then, Dixson released the sharks one at a time into a pool with two side-by-side streams of flowing water. In one stream, the researchers pumped water, in which a dead squid had been overnight. This water smelled strongly of the squid. The other stream contained ocean water with no scent（气味）. Because each stream flowed at the same rate and in the same direction, the squid smell stayed in its stream. The sharks could choose to swim in either stream.

Sharks that had been swimming for five days in pools of regular ocean water spent more than 60 percent of their time in the squid-scented stream. The same was true for sharks that had been in water with the carbon dioxide level that could develop by the year 2050. But sharks exposed to the highest level of carbon dioxide spent just 15 percent of their time in the squid-scented stream.

1.Why did the writer mention the sharks smell at the beginning of the passage?

A. Because the writer appreciates their ability to find food.

B. Because the writer considers them dangerous to human beings.

C. Because the writer intends to show how to control weed in the ocean.

D. Because the writer worries about the sharks in the ocean.

2.What is Paragraph 3 mainly about?

A. What the ocean in the future will be like.

B. Who helped Dixson conduct the study.

C. What kinds of pools were used during the study.

D. What the levels of carbon dioxide will be at in the ocean in the future.

3.The researchers pumped water where a dead squid had been overnight to ______.

A. help the sharks look for food

B. test the sharks' sense of smell

C. prove that sharks like eating squids

D. explain why carbon dioxide in the ocean affects sharks

4.What does the underlined sentence in the last paragraph suggest?

A. Sharks like the squid-scented stream better.

B. Sharks like the stream with the high level of carbon dioxide.

C. Sharks spent 15% of their time in the squid-scented stream.

D. The high level of carbon dioxide can affect the sharks ability to smell.