Chapter 4 的第一题  为什么B不对呢？谢谢！

Which PROC PRINT stepbelow creates the following output?
  题目图表见附件  

a.	proc print data=flights.laguardia noobs; var on changed flight; where on>=160;run;
b.	proc print data=flights.laguardia; var date on changed flight; where changed>3;run;
c.	proc print data=flights.laguardia label; id date; var boarded transferred flight; label boarded='On' transferred='Changed'; where flight='219';run;
d.	proc print flights.laguardia noobs; id date; var date on changed flight; where flight='219';run;

Correct answer:  c