Bowling is an indoor sport. In this game, a player rolls a heavy ball down a long lane and tries to knock down as many pins as possible. Bowling can be played by people of nearly all ages. If a player rolls the ball and knocks down all the pins on the first roll, the player scores a strike. If the ball knocks down all the pins on the second try, the player gets a spare. Both strikes and spares earn extra points I. What is this selection mainly about?o