I would try to eat more healthily because eating certain foods are a huge magnet to toxic waste and gas. This creates more pollutions and destroys the o-zone. Another thing it to take my bike instead of car or use a hybrid or an electric car if it is to far ( my destination) for a bike. Cooking food in my house with an oven is MUCH better for the clean air as well for the coal and charcoal used for grill, and wood for fires, causes a massive pinpointed area of smoke and destroy the air causing bug leaks of hydrogen in the air molecules.
I think it is skin cells and blood cells because sperm and egg cells are haploid, so they only have one copy. I’m honestly not 100% sure tho
But this rule rule is applicable only for double stranded DNA molecule. The possibility of Adenine=40% and Thymine=60% is only in single stranded DNA molecule. In the question, the DNA sample referred is a single stranded DNA. hope it helps :D