Source S is 18 m from a reflecting surface; speed of sound at 30°C is about 349.85 m/s. Will an echo be heard at S?

• ayes, because round-trip time $\approx 0.103$ s exceeds 0.1 s
• bno, because round-trip time is less than 0.1 s
• cno, because the velocity is too low
• dyes, because the one-way time exceeds 0.05 s