编辑幻灯片时，允许撤消的操作次数()。

The teacher would use _to help students communicate in teaching speaking.

提出“究天人之际，通古今之变”治史思想的史学家是()。

不属于幼儿园教师工作主要职责的是( )。

Which one does not belong to subjective questions in the following English tests?