a sound wave is produced fron a source ans an echo is heard t seconds afterwards. If d is the distance of the reflecting surface from the source, v the speed, p the wavelength and T the period of the wave, then

• A.d = \(\frac{2t}{\lambda}\)
• B.d = \(\frac{\lambda t}{2T}\)
• C.d = \(\frac{2T}{\lambda t}\)
• D.d = \(\frac{\lambda T}{2t}\)