求解释sas50题的第29题

.A frequency report of the variable Jobcode in the Work.Actors data set is listed
below.
Jobcode Frequency Percent Cumulative
Frequency Cumulative
Percent
Actor I2 33.33 2 33.33
Actor II 2 33.33 4 66.67
Actor III 2 33.33 6 100.00
Frequency Missing = 1
The following SAS program is submitted:
data work.joblevels;
set work.actors;
if jobcode in ('Actor I', 'Actor II') then
joblevel='Beginner';
if jobcode='Actor III' then
joblevel='Advanced';
else joblevel='Unknown';
run;
Which of the following represents the possible values for the variable joblevel in the
Work.Joblevels data set?
a. Advanced and Unknown only  

b. Beginner and Advanced only
c. Beginner, Advanced, and Unknown
d. ' ' (missing character value)
Correct answer: a

刚接触sas，没太看懂解释，求知识点详细一点的解释

if jobcode='Actor III' then
joblevel='Advanced';
else joblevel='Unknown';
run;
这几行语句已经把jobcode的所有情况都包括了，所以joblevel='Advanced'或者joblevel='Unknown'