请教各位大神第111道题，
A SAS PRINT procedure output of the work. levels data set is listed below:
obs name level
1 Frank 1
2 Joan 2
3 Sui 2
4 Jose 3
5 Burt 4
6 Kelly .
7 Juan 1
The following SAS program is submitted:
data work.expertise;
set work.levels;
if level = . then
expertise ='Unknown';
if level = 1 then expertise ='low';
else if level = 2 or 3 then
expertise='Medium';
else
expertise='High';
run;
which of the follwing values does the variable EXPERTISE contain?
A Low,Medium, and High only
B Low,Medium, and Unknown only
c Low,Medium, High and Unknown only
D Low,Medium, High and Unknown and ' ' missing character value
答案选B，为什么啊，求助~~先谢谢了~~