Regarding question #16 - technically, it can have two correct answers. (and some more) Along the obvious right answer A, stars can and do fuse heavier elements in their cores. Red supergiants go through a carbon fusing (or "carbon burning") phase - coal being primarily made out of carbon makes answer C right as well - some stars actually release energy from burning coal (not exactly, but I find it funny. Also, answer C for Carbon - am I seeing meta jokes where there are none? I probably need some sleep) Also, there are other burning phases with net energy release: helium, neon, oxygen, silicon and iron.