For the most economical trapezoidal section of a channel with regards to discharge, the required condition, is

• A. half of top width = sloping side
• B. hydraulic depth = half the depth
• C. <img src="/_files/images/civil-engineering/hydraulics/155-7.182-1.png">
• D. perpendiculars drawn from the centre of the top width on to the bottom sloping side, are all equal
• E. all the above.