沾锡过程中的锡点大头的原因是（）。

A.导体脏污

B.铜丝未扭紧

C.沾锡速度慢带锡多

D.氧化物附着在导体上

The first walk into outer space was taken by a Soviet astronaut. Astronaut Zhai Zhigang became the first Chinese to walk in space on September 27, 2008. It showed that it was quite safe for a man to be outside the spaceship for a certain period of time. But to walk in space, astronauts have to meet with many kinds of difficulties and dangers.
The spaceship goes round the earth at an altitude of about 200 kilometers above its surface. At such altitudes there is no atmosphere which protects the life on the earth. The conditions of outer space are very harmful to man.
The boiling point of liquids goes down with pressure. But pressure outside the spaceship is nearly zero. In such conditions the decompression of the spacesuit may cause the boiling of the blood and quick death. There is radiation from space and it is harmful.
Breathing problems in space can be solved with the help of the spacesuit. Another problem is heat. Man gives off heat in the course of his activity, and the harder the labor is, the greater the amount of heat is. In space unless special measures are taken, the heat inside the spacesuit would make the conditions difficult.

将划线的句子翻译成汉语。