The overriding problem with both theories of muscle growth, hypertrophy and hyperplasia, is that __________.

The main difficulty is studying muscle growth directly in humans, so it is hard to determine how much growth comes from hypertrophy (larger muscle fibers) versus hyperplasia (more muscle fibers). This makes it difficult to accept either theory as definitively correct. Choice A describes that limitation; the other choices incorrectly claim the theories are impossible, unstudied, or rejected by the fitness community.

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