In an experiment to measure the speed of sound in air, a boy stands 40 m away from a wall and bangs two pieces of wood together. At the instant he hears the echo, he bangs them together again. He does this activity 50 times. The time taken for 50 bangs is 12 s. Which calculation gives the speed of sound in air?

• A. \[\frac{12}{40\times 50}\]
• B. \[\frac{40\times 50}{12}\]
• C. \[\frac{40\times 2\times 50}{12}\]
• D. \[\frac{40\times 2\times 12}{50}\]