Answering one question would have forced him to answer others.
The fifth amendment does not have such a condition in its wording. Although it may be a common practice to take the fifth on all questions, I have not found any law which says this is a requirement otherwise the person is no longer protected by this right. In other words, I think a person can indeed be selective in which questions he pleads the fifth.
----- "We simply attempt to be fearful when others are greedy and to be greedy only when others are fearful." -- Warren Buffett