A. drainage passes over the canal and F.S.L. of the canal is below the bottom of the drainage trough B. drainage passes over the canal and F.S.L. of the canal is above the bottom of the drainage trough C. canal passes over the drainage and H.F.L. of the drainage is above the bottom of the canal trough D. canal passes over the drainage and H.F.L. of the drainage is below the bottom of the canal trough.