adv 63题中45 到底应该选c还是d？多谢

Item 45
To create a list of unique Customer_Id values from the customer data set, which of the following techniques can be
used?
technique 1: proc SORT with NODUPKEY and OUT=
technique 2: data step with IF FIRST.Customer_Id=1
technique 3: proc SQL with the SELECT DISTINCT statement
A. only technique 1
B. techniques 1 and 2
C. techniques 1 and 3
D. techniques 1, 2, or 3

因为 这个 网页的解释就越来越糊涂了。
https://support.sas.com/documentation/cdl/en/lrdict/64316/HTML/default/viewer.htm#a000202968.htm   

当然是C 单用2是不行的 需要先proc sort 然后才能first.varaiable