I heard that it's online based practice tests. But do they have explanations or just tests? which one of these were known as "u uninstall, u can't install again" problem? which one is better between these two if i have to choose one? ppl r keep saying Schaum's bio is good but they never said anything about Schaum's chemistry and organic chemistry. why is that? what do u think is a good study aid for each section? thank you.