53.The following SAS program is submitted:

   data WORK.TOTAL_SALARY;

     retain Total;

     set WORK.SALARY;

     by Department;

     if First.Department

        then Total=0;

     Total=sum(Total, Wagerate);

     if Last.Total;

  run;

What is the initial value of the variable Total?

     A. 0

     B. Missing

     C. The value of the first observations Wagerate

     D. Cannot be determined from the information given

Answer: B

求大神解析一下，retain 的意义和用法！以及本程序答案的解析！

最后想问问，base 70题有没有比较详细的解析啊！我这个版本只有简单的答案！表示非常费解！