根据食品卫生标准要求或对标本污染情况的估计，每次选择（）个适宜稀释度做菌落总数。
A、1～2
B、2～3
C、3～4
D、4～5
参考答案：B
产生正弦波自激震荡的稳定条件是（）。
A、引入正反馈
B、|AF|≥1
C、AF=1
She several newspapers but couldn't find the report about the accident. [ ]
A.looked at
B.looked for
C.looked after
D.looked through
