《中国药典》(2000年版)规定检查N，N-二甲基苯胺特殊杂质的药物是A．青霉素钾B

我国《保险法》规定，交足两年以上保费的保单才有现金价值，但( )条款是特殊的。

A．不可抗辩条款

B．年龄误告条款

C．中止、复效条款

D．自杀条款

在磁感应强度为B₀，方向竖直向上的匀强磁场中，水平放置一根长通电直导线，电流的方向垂直于纸面向里．如图所示，a、b、c、d是以直导线为圆心的同一圆周上的四点，在这四点中（　　）

A．b、d两点的磁感应强度相同

B．a、b两点的磁感应强度不同

C．c点的磁感应强度的值最小

D．b点的磁感应强度的值最大

化脓性中耳炎耳源性颅内外并发症最常见的侵犯传播途径是：（）

A.血行途径

B.循破坏、缺损的骨壁

C.正常的解剖途径

D.尚未闭合的骨缝

E.上述都不是

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.

18（）

A.controls 

B.enemies 

C.choices 

D.means