(2013·北京市西城一模)All the books ________ by the students are reported to have been sent to the children in the countryside the other day.

A．having offered

B．to be offered

C．offering

D．offered

某中等氟病区的病人，其症状为肉眼可见牙齿微黄，并有部分人的牙齿缺损，骨关节疼痛，四肢僵直，进而行动困难。化验：尿氟、发氟增高。

预防氟中毒的最有效的方法是（）。

A.增加营养

B.饮水化学除氟

C.饮水物理除氟

D.改用低氟水源及低氟煤

E.改用低氟煤