You can hardly imagine the once clear canal _____ so seriously polluted that no fish can live in it. [ ]

A. have become  

B. having become  

C. to have become  

D. to become

对于各种氟化物防龋方法的可行性，正确排列为（）

A.自来水氟化》学校口服氟片》学校饮水氟化

B.自来水氟化》家庭口服氟片》含氟牙膏

C.学校饮水氟水》含氟牙膏》涂氟

D.家庭口服氟片》涂氟》含氟牙膏