Question

________ are formed when groundwater dissolves cavities into limestone.

Answer 1

The answer is Caverns

as Caverns of limestone is a natural cavity, which forms in the subsurface of the earth by the effect of acidic groundwater and underground water (like rivers) making the limestone dissolve by time and leaves this cavity. which increase and grow up with time.
and there are many structures which form in this caverns ex : 
(Speleothem, Stalactite, Flowstone, Stalagmite, Helictite)

Answer 2

Answer:

Caves.

Explanation:

Hello,

In this case, caves are formed by the dissolution of limestone. It turns out that rainwater picks up carbon dioxide from the air and as it percolates through the soil it turns into a weak acid (carbonic acid). This causes a slowly dissolution of the limestone along the joints, bedding planes and fractures, some of which become enlarged enough to form caves.

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