for 1 i put b, as the isotope decays by emitting alpha 4/2He but d is ambigious because if u have too many electrons then u have the same no. of protons. oh well
i think 15 is b, but i put the wrong thing down cos when i was working backwards i wrote 0.6 instead of 0.3 on one of the atoms! damn!
14 is put d, cos the river source is nice and clean, then the limestone quarry could introduce calcium carbonate into the water, thus making a buffer and making the water neutral, and then the town has all crap n stuff in the water, thus there is a higher BOD.