can not 和 be unable to

noven-wang

70. A President entering the final two years of a second term is likely to be at a severe disadvantage and is often unable to carry out a legislative program.
(A)    likely to be at a severe disadvantage and is often unable to
(B)    likely severely disadvantaged and often unable to
(C)    liable to be severely disadvantaged and cannot often
(D)    liable that he or she is at a severe disadvantage and cannot often
(E)    at a severe disadvantage, often likely to be unable that he or she can
In C and D, cannot often carry out suggests that a President with limited time suffers only from an inability to achieve legislative goals frequently, not from a frequent inability to achieve them at all
这句话里面似乎暗含着cannot与be unable to的区别的玄机，但是，我有读不出来，烦请大牛们给予解答，而且，ets似乎很偏爱be able to更甚于can，是吗？？

dreadpower

我的看法是：除去cannot（可能）和unable（能力）的区别外，
      cannot often意思是不能经常，间或还能
      be often unable to意思是经常不能，（几乎不可能）
      程度不同

solome

can 侧重于主观的不愿，没有能力
be able to 是外部条件的不容许而无法做某事
他们两个有不同的侧重点
本题就是不是president不能而是外部条件不好而让他不能够carry out a legislative program.

ets是can>be able to的