按《地表水和污水监测技术规范》(HJ/91-2002),现场采集的水样分为()和(),混合水样又分为()和()。
参考答案:瞬时水样;混合水样;等比例混合水样;等时混合水样
It was on the festival occassions __________ the family got together __________ he missed his family.
A.where ; that
B.where; when
C.when; that
D.that; when
交变电流u=50sin100πt(V)的( )
A.周期为100s
B.频率为50Hz
C.电压峰值为502V
D.电压有效值为50V