An observer receives waves directly from a source of sound distant $120\,m$ in a big hall.  He also receives waves reflected from the mid-point of $25\,m$ high ceiling. The wavelength of sound for constructive interference to take place between two waves,  must be :
  • A$5, 2.5, \frac{5}{3}...... m$
  • B$10, 5,\frac{10}{3}...... m$
  • C$20,\frac{20}{3}, 4 ...... m$
  • D$10,\frac{10}{3}, 2 ...... m$
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