I like reading, so I always spend my pocket money ________ books.
要从4名女生和2名男生中选出3名学生组成课外学习小组,则是按分层抽样组成的课外学习小组的概率为( )
“日出江花红胜火,春来江水绿如蓝”,诗中使江水变绿的生物是( )