chemistry and biology question need helps

For question 11, the answer is B. I understand why it is B but I am still curious in knowing what "acidification" means. Can someone please explain what acidification is in this particular context.

For questions 24, the answer is C. Can someone please explain how they would work this out.

Can't really help with the first one, but for the leg one, just flex some muscles in your leg and see what happens to the part below.
1) At the ankle, the foot rotates backwards (go on tip toes), so in the lower leg the calf T contracts / tenses and shin S extends / relaxes
2) At the knee, the lower leg rotates forward, so in the upper leg R contracts and P relaxes
3) At the butt, the upper leg rotates backwards, so Q must contract

There is a hint in picture, fatter muscles in the 2nd image compared to the 1st have contracted and thinner muscles have relaxed. However, the right way to think about it is which way does the joint (lower body part) rotate, so which muscle must contract and as muscles work in pairs, the other must relax.