氟化水源防龋的标准浓度是（） A.0.05～0.1ppm B.0.1～0.5ppm

简述阴虚证与阳虚证的临床表现。

下列现象或事实，用相关知识解释不正确的是（　　）

A．A

B．B

C．C

D．D

Our head teacher saw the empty seat next to me and asked _______. [ ]

A. where has Jeff gone

B. where did Jeff go

C. where Jeff has been

D. where Jeff had gone

理念：行动

A．哲学：席位

B．文学：创作

C．航标：航行

D．文化：传统

简述分馏塔启动前的操作步骤？