DNA变性时其结构变化表现为（)。A．磷酸二酯键断裂B．N-C糖苷键断裂C．戊糖内C-C键断裂D．碱基内C-C

A、produces

B、manufactures

C、does

D、makes

The Cold Places

The Arctic is a polar region. It surrounds the North Pole.

Like Antarctica, the Arctic is a land of ice and snow. Antarctica holds the record for a low temperature reading--125 degrees Fahrenheit below zero. Reading of 85 degrees below zero is common in both the Arctic and Antarctica. Winter temperatures average 30 degrees below zero in the Arctic. At the South Pole the winter average is about 73 degrees below zero.

One thing alone makes it almost impossible for men to live in Antarctica and in parts of the Arc tic. This one thing is the low temperature--the killing chill of far North and the polar South.

To survive, men must wear the warmest possible clothing. They must build windproof shelters. They must keep heaters going at all times. Not even for a moment can they be unprotected against the below-zero temperatures.

Men have a way of providing for themselves. Polar explorers wrap themselves in warm coats and furs. The cold makes life difficult. But the explorers can stay alive.

What about animals？ Can they survive？ Do we find plants？ Do we find life in the Arctic and in Antarctica？ Yes, we do. There is life in the oceans. There is life on land.

Antarctica, as we have seen, is a cold place indeed. But this has not always been the case. Expedition scientists have discovered that Antarctica has not always been a frozen continent. At one time the weather in Antarctica may have much like our own.

Explorers have discovered coal in Antarctica. This leads them to believe that Antarctica at one time was a land of swamps and forests. Heat and moisture must have kept the trees in the forests alive.

The lowest temperature that man has ever known was recorded in Antarctica.

A．Right

B．Wrong

C．Not mentioned

The climate is gradually becoming (dry) ______ and warmer.

England is a country______.

A．with a cold and wet climate

B．with a surprising climate

C．with a pleasant climate

D．with a variety of climates

England is a country ________.

A．with a cold and wet climate

B．with a surprising climate

C．with a pleasant climate

D．with a variety of climates

A．Moderate temperature, relatively dry and ice-free.

B．Extremely cold, relatively dry and ice-free.

C．Extremely cold, plenty of raining and ice free.

D．Extremely cold, relatively dry and ice frosted.

According to the stranger, in the West the winters are______.

A. dry and cold

B. warmer than in the East

C. wet and cold

D. mild