Matching time periods:

You certainly can't assume any information that's not provided in the question in GMAT DS. I don't see how I've done that, however. If I tell you 'I'm free between 10am and 2pm today', that doesn't mean 'I'm *only* free between 10am and 2pm today'. If the question designer intends to convey, in each statement, that the Commissioners are only free at the times given, the question needs to make that clear, and now it doesn't. If that's the intended meaning, then the answer is clearly B, of course.

Real GMAT questions are never open to interpretation, which is why I said above that the question was not well-designed. As I read it, my interpretation is perfectly logical, but it makes the question completely uninteresting, and I'm certain the question designer intended a different interpretation. Of course, that the designer thinks the answer is C makes me wonder if the question designer intended a third interpretation that none of us has discovered yet, but the purpose of GMAT questions is not to test whether we can guess the intentions of the question maker - the correct interpretation is always clear from the wording of the question.