Check all that are true about waterfalls a. water falls move towards their source b. waterfalls move towards their mouth c. the top or cap rock is resistant to erosion d. the lower rocks are resistant to erosion e. water falls indicate a mature river f. waterfalls indicate a youthful river

Waterfalls are created when a river flows following a descending rapid slope. The waterfall, then, flows from the source (where it starts) to the mouth (where it ends).
Waterfalls are created when the erosion of the rocks at the bottom of the slope is more powerful than the erosion of the rocks on the top.
After many years the water is able to erode the rocks on the top as well, and the waterfall slowly disappears.

Therefore the options that apply are:
b) waterfalls move towards their mouth;
c) the top or cap rock is resistant to erosion;
f) waterfalls indicate a youthful river

The answers would be: 1: the liquid only contains one compound4: the liquid is a pure substance