They'll have a beer together______,talking about their separate life, but not often.

A．by and by

B．now and then

C．all the time

D．more or less

某中等氟病区的患者，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、发氟均增高。

氟中毒的机制主要在于（）

A.破坏了钙磷代谢

B.氟磷灰石被取代

C.骨钙被取代

D.钙质在骨内过多沉积

E.碱性磷酸酶的破坏