A detector emits one pulse at a time. Each echo must return before the next pulse is emitted. The greatest distance to the reflecting surface is 75 m and the wave speed is 1500 m/s. Ignoring pulse duration, what is the maximum number of pulses emitted per second?
40 s−140\ \mathrm{s^{-1}}40 s−1
5 s−15\ \mathrm{s^{-1}}5 s−1
10 s−110\ \mathrm{s^{-1}}10 s−1
20 s−120\ \mathrm{s^{-1}}20 s−1