Which one of the following is true of the RETAIN statement in a SAS DATA step
program?
A. It can be used to assign an initial value to _N_ .
B. It is only valid in conjunction with a SUM function.
C. It has no effect on variables read with the SET, MERGE and UPDATE statements.
D. It adds the value of an expression to an accumulator variable and ignores missing
values.
Answer: C
请解释一下为什么选C，其他几项错在哪里，谢谢