Concept:A meander is a loop or bend in a river. Meanders form where the river flows slowly over gentle, almost flat land.Solution:In the lower course the gradient is very gentle, so the river flows slowly and erodes sideways (lateral erosion) instead of downwards.The current erodes the outer bank and deposits material on the inner bank, so the bends grow into meanders.A steep valley (B) and vertical erosion (C) belong to the upper course. Deposition is high, not low (D), in the lower course.Answer:Option A (valley is almost flat)