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

背景资料：

某公路隧道是单洞双车道形式，开挖的单圆半径尺=5.6m，直立边墙高为3m，要求采用新奥法施工，进行喷锚等形式的初期支护。现有两座位于紧靠乌江一侧的高山隧道工程，1号隧道里程是K5+100一K6+320，2号隧道里程是K6+400一K6+750。1号隧道的进口处K5+100可以进洞施工，有工作面。l号隧道的出口处和2号隧道的进口处无法专门修建临时道路形成工作面进洞，必须从2号隧道的出口处K6+750进洞施工，打通2号隧道后，出了2号隧道的进口，再从l号隧道的出口处K6+320进洞施工。工程开工时间为施工当年的5月15日。某国有公路工程公司中标，负责该工程的施工。公路工程公司规定在第二年的3月15日实现两座隧道的上台阶全部掘进贯通，公司对整个隧道工程作了如下分工和要求：

（1）公司的第一工程处负责1号隧道的K5+100~K5+900，长度为800m，从隧道的进口处进洞施工；

（2）公司的第二工程处负责l号隧道的K5+900~K6+320和整个2号隧道，共计770m长，从2号隧道的出口处K6+750开始进洞施工；

（3）两个工程处施工的工程分界线位于1号隧道的K5+900处。

为了保证工程按时完工，当规定的两座隧道上台阶全部贯通时间到达时（即第二年3月15日），先到达分界线的工程处，可以趱过分界线将另一工程处的工程作为自己的工程进行施工，直到贯通为止，所增加隧道长度的工程量将纳入该工程处的工程费用中。因此作为该公司的第二工程处必须抢先打通2号隧道以保证l号隧道的正常施工。在2号隧道施工过程中发生了以下事件：

事件一：为了抢进度和节约成本，在2号燧遵施工中未使用J通风设备，在掘进250m（即K6+500）时，施工人员感觉缺氧、头疼、呼吸困难，希望项目部提供通风设备。而项目部鼓励大家克服困难尽快贯通2号隧道，并许诺给予奖励，但如果施工人员擅自停工或怠工将采取经济处罚。

事件二：当2号隧道继续施工掘进330m（即K6+420）接近隧道贯通时，有6名施工人员昏倒在施工现场，经抢救只有2人生还，4人死亡。项目经理未及时向公司报告人员伤亡情况，以项目部名义与伤亡家属协商赔偿事宜。由于死者家属不满意赔偿金额，项目部才向公司报告有4人死亡。公司主要负责人得知此事2小时后将4人死亡的事故上报当地政府的职能部门。

施工单位违反了《安全生产法》第五十一条从业人员的拒绝权利和强令员工冒险作业的规定。