题干
— Was his father very strict with him when he was at school?
— Yes. He never praised him ________ he became one of the top students in his grade.
A：after
B：unless
C：if
D：when
答案(点此获取答案解析）
B
的图象如图1所示，它在定义域上是减函数，给出如下命题：①
=1；②
；③若
，则
；④若
，则
，其中正确的是（ ）