Concept:Irregular water levels in rivers hinder navigation, and engineering solutions help maintain a usable waterway.Explanation:The main way to overcome irregular water levels is by controlling water depth and flow through artificial structures.Locks raise or lower boats between stretches of water at different levels.Canals provide alternative routes where river levels are unreliable or obstructed.Thus, building locks and canals solves the problem.Answer:B. building locks and canals