They may have ran out of oxygen, or they may not have been water dwelling snails, as not all are, lack of food, no flow of water into and out of the tank would cause them to run out of oxygen.
Genetically Modified Organisms, or GMO's are an improvement in efficient agriculture around the world, made possible by biotechnology. People who work in biotechnology worked to create GMO's and they are wildly used all over the world.
The answer is D increased precipitation
