问答题 管电压越高，平均波长越短，射线穿透能力越强的现象叫X射线线质（）。

判断题 After SELECT, only the field name of the table can appear, not an expression.( )

单项选择题 2.I can’t stand him. He always talks as though he___________ everything. A、 knew B、knows C、has known D、had known