试述医院感染的定义.

债务承担可以分为免责的债务承担与并存的债务承担，下列各项属于两者共同点的是（）。

A.原债务人与新债务人一起对债权人承担债务

B.原债务人均脱离债的关系

C.原债务人均不脱离债的关系

D.新债务人可以主张原债务人对债权人的抗辩

一个圆锥形沙堆，底面积是45.9m²，高1.2m。用 这堆沙在12m宽的路面上铺3cm厚的路基，能铺多少米？

侵蚀性葡萄胎多发生在葡萄胎排空后（）

A.3个月内

B.5个月内

C.6个月内

D.7个月内

E.12个月内

测直齿圆柱齿轮多采用公法线长度测量，测量时，卡尺与齿面接触处应尽量在（）附近。

A.齿顶高

B.齿根高

C."分度圆`"

D.基圆

It is clear that some chemicals can damage the health of animals and humans. (1) , this is not the only problem that (2) be caused by the careless use of chemicals. Chemicals can also (3) the ecological balance of the environment. If the ecological balance is disturbed, the results can be (4) serious.

The (5) of DDT illustrates the problem. DDT, a chemical which kills insects, at first seemed to be a perfect answer (6) many problems. It would control insects that caused dangerous diseases, (7) insects that caused billions of dollars of damage to crops every year. Governments permitted and even (8) the use of DDT. Farmers in many countries (9) to spray it on their crops. The (10) results were good: Damage to crops (11) down, and profits went up. However, the chemical had effects which the scientists didn’t see in (12) . First, it also killed insects which were the (13) enemies of the harmful insects and which were therefore beneficial to farmers. Second, and perhaps worse, DDT did not kill (14) harmful insect. A few insects had natural resistance to the chemical. They (15) and reproduced in large numbers. In a few years there were large numbers of insects which were not (16) by DDT, and there were (17) insects which could act as natural (18) on these new "super-insects". Finally, it became clear that DDT was not solving the insect problem. In fact, it was making the problem worse. It (19) became necessary to find a second (20) for the effects of the first.

17（）

A.many 

B.a few 

C.a little 

D.fewer