Right! The answer is C) Three basic types of sinkholes.
The kind we see in Florida is called a cover-collapse sinkhole. It tends to occur in clay, because clay holds soil together like glue. As soil leaches into a cave below the rock, it creates a void in the soil that moves upward. You can't see it on the surface. Then, all of a sudden, the bridge over top of that void can't hold anymore and it collapses.
Another kind is called a cover-subsidence sinkhole. In places with sandier soil and a void underground, as the soil above transports itself into that cave in the rock, the ground slowly subsides. So it's not catastrophic. It subsides over time. It could be over years, or even hundreds of thousands of years.
Then there are solution sinkholes, which occur in areas where limestone is exposed at land surface, or is covered by thin layers of soil and permeable sand. This type of sinkhole usually forms as a bowl-shaped depression with the slope of its sides determined by the rate of subsidence relative to the rate of erosion of the walls of the depression from surface runoff. Surface runoff may also carry sand and clay particles into the depression, which may form a relatively impermeable seal in the bottom.
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