Children get tired of learning often because they _____ to do better than they can both at school and at

home.[ ]

A. hope        

B. are hoped      

C. expect        

D. are expected

为了解某校近视的发生率，调查者计划从全校50个班中，随机抽取5个班，然后调查这些班中的所有学生，这种抽样方法称为

A．单纯随机抽样
B．系统抽样
C．整群抽样
D．分层抽样
E．机械抽样