practice test is quite different from reading LSAT materials. Think about the time constraint. For my first practice test, I stopped in the middle because I felt like I was rushing to the end rather than confidently answering questions. If I had finished the test, I would likely have scored <140. You can definitely see the improvement if you give it enough time.

Thanks a lot for the reply. You are definitely right about "A would be useless if B isn't true; But B can still support the argument without A being true". And I guess the question is in the end about relevance. If I can allow "proper spine alignment" irrelevant even if it is written in the argument and supports MP/SC, then A doesn't matter. But I guess I can't allow "balanced muscle development" to be irrelevant since it is in the MP/SC to directly support the MC. So B must be true. Thanks again for the reply!

I am still hesitant about choice A and B. A links the idea between proper alignment of the spine and balanced muscle development. B links the idea between health exercise equally and balanced muscle development. I think Either A or B is sufficient to make the argument valid: A links P and MP/SC and MC together while B directly links MP/SC and MC. In the end I chose B only because it's more relevant, which makes it more attractive for a necessary assumption. Any comments would be greatly appreciated.