在社区卫生工作中操作起来最为方便的抽样方法是（）
A.单纯随机抽样
B.整群抽样
C.机械抽样
D.分层抽样
E.雪球抽样
参考答案：B
It’s the second time that I in Beijing. What great changes! It’s years since I
here last time.
A．went; had left
B．had been; left
C．have been; left
D．have; have left
木模板宜在安装前涂刷适宜的隔离剂，不得在安装后涂刷，以免污染钢筋和混凝土。( )
