Final answer:
The indication that erosion was caused by a waterfall is the presence of a plunge pool. A flat base of a river and curving rivers can also be associated with waterfalls but are not direct indications.
Step-by-step explanation:
The indication that erosion was caused by a waterfall is the presence of a plunge pool. A plunge pool is a pool of water formed at the base of a waterfall as a result of the erosive force of the falling water. It is deeper and usually wider than the surrounding river channel.
On the other hand, a flat base of a river is more likely to indicate erosion caused by a meandering river rather than a waterfall. A curving river, although not directly indicating a waterfall, can often be associated with the presence of waterfalls as they occur in areas of varying terrain and can cause the river to take a winding course.
An infinity pool, in contrast, is a type of swimming pool and does not indicate erosion caused by a waterfall.
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