The main function of a divide wall is to A. control the silt entry in the canal B. prevent river floods from entering the canal C. separate the undersluices from weir proper D. provide smooth flow at sufficiently low velocity

The correct answer is: C. separate the undersluices from weir proper.

A divide wall is a structure that is built to separate two different watercourses. In the case of a canal, the divide wall separates the undersluices from the weir proper. The undersluices are used to control the flow of water through the canal, while the weir proper is used to create a head of water for irrigation or other purposes. The divide wall ensures that the two watercourses do not mix, which can help to prevent erosion and other problems.

Option A is incorrect because the divide wall does not control the silt entry in the canal. This is typically done by using a silt fence or other type of barrier.

Option B is incorrect because the divide wall does not prevent river floods from entering the canal. This is typically done by using a flood control gate or other type of barrier.

Option D is incorrect because the divide wall does not provide smooth flow at sufficiently low velocity. This is typically done by using a baffle or other type of structure.