10. The SAS data set Sashelp.Prdsale contains the variables Region and Salary with 4 observations per Region. Sashelp.Prdsale is sorted primarily by Region and within Region by Salary in descending order.
The following program is submitted:
data one;
set sashelp.prdsale; retain temp; by region descending salary;
if first.region then
do; temp=salary; output;
end;
if last.region then
do; range=salary-temp; output;
end;
run;
For each region, what is the number of observation(s) written to the output data set?
a. 0
b. 1
c. 2
d. 4
为什么这里答案是C呢,求高手帮忙解答一下!