내가 요새 학원그만두고 독학하느라 물어볼사람이 없어서용
도와줘용
 "Students, this material that we\'ve been covering is very important. ___________miss any classes this week, or you will not
   do well on your final exam."
(a)You have to
(b)You dont have to
(c)You\'d better not
(d)You\'re obliged to

여기서 답은 (c)인데, 왜 (b)는 안되는거에용??

In the past, Americnas obtained ample energy with a technology ________. Now we depend increasingly on electricity generated
in central power stations.
(A)no more refined than an ax
(b)highly developed
(c)imported entirely from overseas
(d)related with nuclear energy

이거 답은 (a)인데 답이 도대체 어디서 유추가 되는거죠? 해석을 봐도 이해도 안되공-_-;;

고수님들아 도와줘용.....