OG SC70

SCもう一問よろしくお願いします。

OG SC70
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
(C)liable to be severely disadvantaged and cannot often

(B)(D)(E)は形からすぐ間違いがわかるので省略

OGの解説によると、
(A)の is often unable to carry out
(B)の cannot often carry out
では意味が(C)が適切でないから(A)ということでした。

In C, "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.

この意味の違いを説明していただけないでしょうか？
よろしくお願いします。