A:However a serious problem
B:What a serious problem
C:However serious a problem
D:What serious a problem
等比数列an的首项为1,公比为q,前n项的和为S,由原数列各项的倒数组成一个新数列1an,由1an的前n项的和是( )
如图,D是△ABC的边AB上的一点,那么下列四个条件不能单独判定△ABC∽△ACD的是( )