She doesn't know why it is _____ that.[ ]

A. call

B. to call

C. called

D. calling

预防地方性氟中毒主要的措施是（）

A.增加氟的摄入量

B.减少氟的摄入量

C.种植吸氟植物

D.食物除氟

E.治疗氟斑牙