thanks a lot paes...the link has excellent explanation

I don't think the problem lies in the " 'S," but more in the comparison. The correct answer states that a black hole lies at the center OF the Milky Way and OF many of the...other galaxies. In this structure, it is then understood that you're talking about the "center" in both "the Milky way" and also "many of the...galaxies."

Once you throw in the " 'S," you're only talking about the Milky Way's center and referring just to the other galaxies in general, NOT their centers. You would be saying that black holes lie at many of the 100 billion other galaxies, but not that they're in the center of them.