【正确答案】 D

【答案解析】[解析] The dunes called Spirit Sands make up the Manitoba Desert-Canada's only desert. These five kilometers of dunes were formed 10,000 years ago, when an ancient river dumped billions of tons of sand and gravel at the edge of a glacial lake. The dunes of Spirit Sands are constantly changing ... they are truly "rolling" dunes. Here's how it works. The sand in each dune becomes progressively finer toward the top. The heavier particles tend to settle at the base on the windward side. The wind blows the finer particles up the slope, and eventually they kind of trickle down the other side. Thus, the dune sort of walks downwind. It will reverse direction when the wind changes. Each dune is covered with tiny, rolling waves, and each wave itself is a tiny dune. What is the lecture mainly about? The speaker mainly discusses how sand dunes shift position. She says: The dunes of Spirit Sands are constantly changing...; Here's how it works; ... the dune sort of walks downwind," It will reverse direction