Pick up the correct statement from the following: A. The water supply pipes carry pure water free from solid particles B. The water supply pipes get clogged if flow velocity is less than self cleansing velocity C. The sewers may be carried up and down the hills and valleys D. The sewer pipes are generally laid along level hills

The correct answer is: B. The water supply pipes get clogged if flow velocity is less than self cleansing velocity.

The water supply pipes carry clean water from the treatment plant to the consumers. The flow velocity in the pipes should be greater than the self-cleansing velocity to prevent the accumulation of solid particles and clogging. The self-cleansing velocity is the minimum velocity required to keep the pipes clean. If the flow velocity is less than the self-cleansing velocity, the solid particles will settle down on the pipe walls and form a layer. This layer will gradually increase in thickness and eventually block the pipe.

The other options are incorrect. Option A is incorrect because the water supply pipes may carry some solid particles, such as minerals and sediment. Option C is incorrect because the sewers are designed to carry wastewater, which is a mixture of water and solid waste. Option D is incorrect because the sewer pipes are generally laid along the contours of the land, which may include hills and valleys.