The correct options are
A Rapids
D Waterfall
A river's fast-flow in the mountainous region causes formation of rapids. The fast-flowing water in a stream is called a rapid. Often, waterfalls form as streams flow from soft rock to hard rock. This happens both laterally (as a stream flows across the earth) and vertically (as the stream drops in a waterfall). In both cases, the soft rock erodes, leaving a hard ledge over which the stream falls.