A) single-gene and chromosomal disorders, infection, malnutrition, poisoning, and problems before or during birth. B) eating too much chocolate, not exercising, and not drinking enough. C) watching too much television, exposure to secondhand smoke, and drinking cow's milk before the age of six months. D) inheriting a very low IQ, not having mobiles in the crib or being read to, and being ignored. E) being exposed to certain bacteria, fungal spores, or mouse excrement.