最近在做50题，碰到一道题不太明白，还请大家指点。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 I 2 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)


为啥选a类？请详细解答一下
多谢！！！！