单选题All we have a clock located inside our brains. Similar to your bedside alarm clock, your internal clock runs on a 24-hour cycle. This cycle, called a circadian rhythm, helps control when you wake, when you eat and when you sleep. Somewhere around puberty, something happens in the timing of the biological clock. The clock pushes forward, so adolescents and teenagers are unable to fall asleep as early as they used to. When your mother tells you it"s time for bed, your body may be pushing you to stay up for several hours more. And the light coming from your computer screen or TV could be pushing you to stay up even later. This shift is natural for teenagers. But staying up very late and sleeping late can get your body"s clock out of sync with the cycle of light and dark. It can also make it hard to get out of bed in the morning and may bring other problems, too. Teenagers are put in a kind of a gray cloud when they don"t get enough sleep, says Mary Carskadon, a sleep researcher at Brown University in Providence, RI. It affects their mood and their ability to think and learn. But just like your alarm clock, your internal clock can be reset. In fact, it automatically resets itself every day. How? By using the light it gets through your eyes. Scientists have known for a long time that the light of day and the dark of night play important roles in setting our internal clocks. For years, researchers thought that the signals that synchronize the body"s clock were handled through the same pathways that we use to see. But recent discoveries show that the human eye has two separate light-sensing systems. One system allows us to see. The second system tells our body whether it"s day or night.

单选题听下面一段对话，回答下列问题。

单选题Why didn't you make me a telephone call yesterday? I ______ about it.

单选题— I called his office, his home, and all other places he ______ possibly go to. But he was nowhere to be found. — He may not be in town.

单选题The words "host and hostess" in the last paragraph of Letter B mean ______.

单选题I'm sorry. I ______ you ______ to me.

单选题Don't punish him too much. ______ he's only a boy of eight.[A] First[B] First of all[C] After all[D] At all

单选题To our delight, she quickly ______ herself to the situation.

单选题What'swrongwiththatcar?[A]Itwasbroken.[B]Itstoppedonthewayformanydays.[C]Itdidn'tstart.

单选题China ______ the developing countries in the world.

单选题Howmanypostofficesdoesthemantalkabout?

单选题It's no use ____________ with him. He won't change his decision.A. arguingB. to argueC. to be arguingD. if you argue

单选题It is reported that the United States uses ______ energy as the whole of Europe. A. as twice B. twice much C. twice much as D. twice as much

单选题It's 11 ______. We'll soon have our lunch. [A] a.m. [B] p.m. [C] clock

单选题How close adults are to their youngsters ______ a strong influence on the development of the children's character.

单选题______ little boy likes to read stories about heroes and wishes to be ______ hero like Dong Cunrui. A.The, the B.The, / C.The, a D.A, /

单选题______ in the traffic jam, he got to school very late this morning.