单项选择题 Almost 20% of all American families ________ single parent families, and in 85% of these families the single parent ________ the mother.A. are … isB. are … areC. is … areD. is … is

单项选择题 钳型电流表只能在()伏电压以下的线路使用。

单项选择题 非线性回归相关指数R的取值范围是( )。