我要选的话，可能会被干扰到B项去，确实没太了解index的作用

the data sets that are listed in the SET statement must be sorted by the values of the BY variable(s), or they must have an appropriate index.BASE书中的原文，也就是要用BY variable语句的话前提是之前对该variable进行了SORT步骤，或者在该VARIABLE本身就是一个索引。所以我做题时AB都想选过两天要去考了，，实在想求一个正确的答案呀。

the data sets that are listed in the SET statement must be sorted by the values of the BY variable(s), or they must have an appropriate index.BASE书中的原文，也就是要用BY variable语句的话前提是之前对该variable进行了SORT步骤，或者在该VARIABLE本身就是一个索引。所以我做题时AB都想选过两天要去考了，，实在想求一个正确的答案呀。

经试验正确答案是D

同理注意45题 给出的答案是C是否也不对

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


C :if first need sort