关于这道问题实在是不太懂也！
A SAS PRINT procedure output of the WORK.LEVELS data set is listed below:

Obs name  level1   Frank 12   Joan  23   Sui   24   Jose  35   Burt  46   Kelly .7   Juan  1The following SAS program is submitted:

data work . expertise;    set work. levels;    if level = . then        expertise = 'Unknown';    else if level = 1 then        expertise = 'Low';    else if level = 2 or 3 then        expertise =' Medium';    else        expertise = 'High';run;

Which of the following 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, Unknown, and ‘ ‘ (missing character value)