请大家帮我看看。给的答案是Ａ，但是我用code验证应当是Ｄ呀　

52. To create a datasetwith unique values of a given varible using a data step and the FIRST. andLAST. varaibales, it is assumed that the input dataset is:   

A. sorted on that variable.

B. indexed by that variable.

C. naturally in order.     

D. any of the above A, B, or C