sas base 70题的第53题原题如下：

The following SAS program is submitted:

  data WORK.TOTAL_SALARY;

     retain Total;

     set WORK.SALARY;

     by Department;

     ifFirst.Department

       then Total=0;

     Total=sum(Total,Wagerate);

     if Last.Total;

  run;

What is the initial value of the variableTotal?

A. 0

B. Missing

C. The value of the first observationsWagerate

D. Cannot be determined from theinformation given

答案：B

答案应该怎么解读？

这里面是因为by 的变量是department，所以last.total的值是miss，是这样吗？