Based on the descriptions of Rock A and Rock B, the most logical conclusion is:
Rock B has been weathered for a longer period of time than Rock A because Rock B is rounder and smoother.
In river beds, rocks typically become rounder and smoother over time due to abrasion from water flow and colliding with other rocks. Therefore, if Rock B appears rounder and smoother than Rock A, it suggests that Rock B has undergone more weathering and erosion.