51. 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;
  [你要选的选项]
run;

问题：which statement completes the program, creates a listing report that is grouped by Id,and completes without errors?
A. by Id;
D.by descending Id;

A是正确答案，结果产生了3个表 A、P、R
但是D为什么结果只产生了P、A两个表啊？


求助大家~~~~