Your professor has asked me to talk to you today about the topic that should be of real concern to civil engineers :the erosion of the US beaches. Let me start with some statistics. Did you know that 90% of the coast in this country is eroding, on the gulf of Mexico for instance, erosion averages 4 to 5 feet per year. Over the past 20 years, there has been an increase in building along the coast, even though geologists and environmentalists have been warning communities about problems like erosion. Some way communities have tried to protect their building and roads and to build seawalls. However geologists have found that such stabilizing structures actually speed up to the destruction of the beaches. These beaches with seawalls, called stabilized beaches, are much narrower than beaches without them. U may wonder how seawalls speed up beach loss. The explanation is simple. If the flow of the beaches is gentle, the water energy is lessened as it washes up along the shore. It is reduced even more that returns to the sea so it doesn't carry back much sand. On the other hand, when the water hit the nearly vertical face of the seawall, it goes straight back to the sea with the full force of its energy and it carries back a great deal of sand. Because of the real risk of losing beaches, many geologists support a ban on all types of stabilizing construction on the shore lines.
解析
What is the speaker mainly discussing?
主旨题。根据…the topic that should be of real concern to civil engineers：the erosion of the US beaches．可知主要讨论的是美国沙滩的侵蚀问题。因此，正确答案为C。