Question

Studying and paying attention in class are critical to success on the final exam. Using the context, the word "critical" means
A. Worthless
B. Necessary
C. Interesting
D. Available

Answer 1

B-necessary.  If you want success on the final exam, then you have to study and pay attention. 
D does not make sense
C critical does not imply interesting at all
A- obviously studying and paying attention are in now way worthless to the cause of the final exam