It Is a Principle of Ethics That "Ought" Implies "Can

It is a principle of ethics that "ought" implies "can." What does this mean?

A) If you can do something, you ought to do it.
B) There are some things you ought to do, but you can't.
C) There are some things you can do, but you ought not.
D) You are only morally responsible for things that you are capable of doing.
E) You are only morally responsible for things that you ought to do.