Which of these activities will most likely impose an external cost? Oa. A construction worker eats a hotdog during her lunch break. Ob. A young father pushes his baby in a stroller. Oc. A postal worker smokes a cigarette in a crowded break room. Od. An athlete works out at a gym. The market demand curve Oa. represents the sum of the quantities demanded by all the buyers at each price of the good. Ob. is found by vertically adding the individual demand curves. Oc. represents the sum of the prices that all the buyers are willing to pay for a given quantity of the good. Od. slopes upward.