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！