A bridge is charted with a vertical clearance of 20 feet referenced to MHW. The predicted height of tide at the time of passage is +2 feet above MLLW, and MHW at this location is 5 feet above MLLW. What is the actual vertical clearance under the bridge?

Why C is correct

The water surface is 2 feet above MLLW. MHW is 5 feet above MLLW, so the water surface is 3 feet below MHW. The charted clearance of 20 feet is measured from MHW, so actual clearance = 20 - (5 - 2) = 20 - 3 = 17 feet. As the tide rises toward MHW, vertical clearance shrinks. (Bowditch Ch. 9 §904)