诸位大侠请看题
the SAS data set work.temp is indexed on the variable id:
id     amount
p       52
p       45
a       13
a       56
r       34
r       12
r       78
the following SAS program is submitted:
proc print data=work.temp;
     [_insert_BY_statement]
run;
which by statement completes the program creates a listing report that is grouped by id. and completes without errors?

A:    by id;     B:    by id grouped;    C:      by id descending;     D:     by descending id;

首选答案应该从A 或者D里面选。但是因为该数据集为 index，所以，在SAS里面运行，二者都带有错误。

请大家帮小弟看看，为什么答案是 A？   非常感谢