题目内容：

What does fizz (气泡) taste like? In Bubbly (多泡的)drinks such as sodas, tiny bubbles give the drink a lift--- and have a distinct taste, In a new study on mice, scientists have connected that fizzy-taste feeling to the ability to taste sourness, such as that of oranges or vinegar.

Scientists first thought the taste of bubbles came from the bubbles bursting on the tongue, but now ate starting to think differently. Charles Zuker, of Columbia University, and his team studied the nervous system of mice to understand how the tongue tastes carbon dioxide, which is the gas that makes up the bubbles.

       Animals, including human beings, are able to detect different tastes by using taste buds(味蕾) which pick up tastes in the mouth, and then send them to the brain. In the experiment, different groups of mice were genetically engineered to be missing one of the senses involved in taste. “Genetically engineered” means the researchers were able to turn off the switches for certain senses by changing the genes responsible for taste. The mice in one group could not taste sweet; another, sour; the third, bitter, and the fourth, salt. When the scientists gave carbon dioxide to the mice, the nervous systems of all the mice responded to the gas, except those of the mice that could not taste sour.

       This shows that the taste of the bubbles must be sour, and that by turning off the ability of the mice to taste sour, the scientists also turned off their ability to taste carbon dioxide. When they studied the cells that detect sourness, the researchers found a protein attached to the cells that is important to the process of tasting carbon dioxide. When carbon dioxide comes into contact with this protein, the protein knocks off particles called protons. These protons(质子), in turn, travel to the brain, which says ,” Hey! That’s a taste!”

       It may seem like a lot of work to get from a can of soda to a taste, but the science of the senses is anything but simple, “  Taste is a challenging system to study,” one researcher says.

1. What is the most important function of the bubbles?

A. To look interesting .                   B. To make drinks taste good.

C. To make drinks funny.                D. To produce a lot of fizz.

2.From the experiment the researchers learned that______.

A. sourness has nothing to do with the taste of bubbles.

B. there is a connection between sourness and bubbles.

C. the taste of bubbles is better if it’s less sour.

D. most mice cannot taste carbon dioxide.

3. It can be inferred from the passage that ________.

A. the taste of bubbles is produced by the bubbles bursting on the tongue.

B. the nervous systems of mice show how the tongue tastes carbon dioxide.

C. taste seems simple but is very complex to research.

D. nerve cells sending signals to the brain is the first step in tasting something.

4. Which of the following would be the best title for the passage?

A. Nervous Systems Understand How the Tongue Tastes.                    

B. The Process of Taste

C. The taste of Bubbles.                                              

D. Different Animals Detect Different Tastes.