单项选择题

以下氟来源中，人体氟的主要来源是（）
A、食物
B、饮水
C、空气
D、含氟牙膏
E、含氟漱口水

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单项选择题

Why shouldn't students take too many things to college with them?
A、Because there isn't often much space in student accommodation.
B、Because it might get stolen.
C、Other students might think they are showing off.
D、It might stop them from focusing on their studies. 无解析

不定项选择

超精密加工的加工精度为1~0.1μm，加工表面粗糙度为0.03~0.005μm

A．正确
B．错误