19.请认真阅读下列短文，从短文后各题所给的A、B、C、D四个选项中，选出最佳选项，并在答题卡上将该项涂黑。

　　Everyone is familiar with white fat, which serves as an energy warehouse. Many mammals also have brown fat, which has the unique ability to produce heat.

　　Brown fat is sometimes mistaken for a type of gland (腺), which it resembles more than white fat. It varies in color from dark red to tan. When the animal is exposed to a cold environment, the color darkens. In contrast to white fat, brown fat has numerous nerves which provide sympathetic stimulation (刺激) to the fat cells.

　　Brown fat is most noticeable in newborn animals, wrapped round the central organs to keep them warm. In human infants it occupies up to 5% of body weight, then declines gradually. Certain quantities of brown fat can be discovered in adult humans, especially when they are exposed to cold temperatures. Most of brown fat in adults is located in the lower neck.

　　Exposure to cold leads to sympathetic stimulation of brown fat cell. As in white fat, sympathetic stimulation promotes hydrolysis (水解) of triglyceride (甘油三酸脂), with release of fatty acids. In contrast to other cells, including white fat cells, brown fat cells express UCP1, which gives the cell’s mitochondria (线粒体) an ability to produce heat rather than ATP. Within brown fat cells, most fatty acids are immediately combined with oxygen in mitochondria and a large amount of heat is produced.

　　There is only about 50g of brown fat in the neck region and it switches on and off throughout the day as it’s exposed to different temperatures or if you exercise or eat. But this capacity is much greater in young children compared with adolescents and adults.

　　The challenge is now to use the knowledge to find out what might switch on brown fat. If it is switched on, we can benefit from it. Brown fat produces lots of heat by burning calories. Unlike white fat, brown fat does not expand our ageing waistlines. And that’s why the race is on to find out more about brown fat, and how humans could use it to our advantage, though we would waste energy unnecessarily, and we would sweat a lot and forever be opening windows.

（1） Which is the unique factor of brown fat producing heat?

A. Fatty acids.                                

B. Triglyceride.

C. UCP1.                                    

D. ATP.

（2） We can learn from the passage that brown fat ______.

A. becomes less in amount with age

B. proves less active in low temperatures

C. exists nowhere but in the lower neck

D. has nothing in common with white fat

（3） The underlined word “infants” in Paragraph 3 means “______”.

A. cells                

B. organs                

C. bodies                

D. babies

（4） We take special interest in brown fat mainly because it helps us ______.

A. save energy                                 

B. develop power

C. keep warm                                

D. keep our figure