Tom buys a pizza and with that same amount of money he could have bought a drink and a hot dog. The opportunity cost is what the the drink.
the money.
the hotdog.
the drink and the hotdog.

According to an observer on earth, planet x is stationary. maria and samir depart earth on their spaceships at the same time, each flying at 0.5c. maria flies toward planet x and samir flies in the opposite direction. according to maria, her voyage to planet x takes 9 years. how many years does maria's voyage take according to samir?