A student is positioned a distance $x$ from a large wall. He claps his hands at a steady rhythm so that every clap lines up with the echo from the one before it. During a time interval $t$, he claps his hands $N$ times. Which expression is used to calculate the speed of sound in air?
- A$\frac{x}{Nt}$
- B$\frac{2x}{Nt}$
- C$\frac{Nx}{t}$
- D$\frac{2Nx}{t}$