27. The Grand Theatre in Shanghai is open to visitors during the daytime ______ there is a performance going on.

实验中的下列操作正确的是                                 (    )

在下列选项中，选出成语书写有误的一项是 （     ）

In my opinion, it is the present examination system rather than the teachers, that _____for the heavy burden on today's students.

《晋书·食货志》记载：“（中原）大疾疫，兼以饥馑，百姓又为寇贼所杀，流尸满河，白骨蔽野。……汉人遭遇亘古未有的劫难，中原成为胡人牧马之所……”据此并结合所学知识可知，中国历史上中原人口南迁的第一次高潮的原因有：（   ）
①中原地区社会动乱；                 ②中原地区出现饥荒和瘟疫；  
③少数民族内迁；                      ④汉族统治阶级内乱