Different chemical reactions are used in combinations during chemical synthesis in order to obtain a desired product. In biochemistry, a consecutive series of chemical reactions (where the product of one reaction is the reactant of the next reaction) form metabolic pathways.
AF-S is AF 'single'- your camera will focus on a fixed object when you press the AF button and will remain focused on that point for as long as you hold the button down.This is best for static subjects.AF-C is AF 'continuous' - the camera will focus on whatever is in the relevant AF points (depending on how you have your AF coverage set up) and will attempt to keep the subject in focus wherever it moves, whether that's towards/away from the camera or laterally.F-A is AF 'auto' and automatically switches between the two AF modes depending on what the subject is doing. This requires less effort on the part of the photographer, but you are relying on the camera guessing correctly what you are focussing on, and may result in unexpected focus.
The AFQT is a grouping of sub-sections of the ASVAB and each branch has different minimums for the AFQT. For the Air Force, you have to score 36 of 99 on the AFQT. For the Army, the minimum is 31 without a waiver. For the Marines, the minimum AFQT is 32 with few waivers granted for lower scores. For the Navy the minimum is 35 and the Coast Guard is 40! I suggest you focus on getting the best ASVAB scores that you can get. Take practice tests and if you get test anxiety take tests over and over so you are familiar with it.