请问这个题要选哪个?为什么,谢谢
Item 52
To create a dataset with unique values of a given varible using a data step and the FIRST. and LAST. variables, it isassumed 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
请问这个题要选哪个?为什么,谢谢