题目

 _____ such heavy pollution already, the river is now too difficult to clean up. A. Suffered         B. suffer                   C. Having suffered      D. To suffer     答案：C河流两岸a、b互相平行，C、D是河岸a上间隔50m的两个电线杆．某人在河岸b上的A处测得∠DAB=30°，然后沿河岸走了100m到达B处，测得∠CBF=60°，求河流的宽度CF的值（结果精确到个位）．