Answer:
The answer is C
Step-by-step explanation:
As the passage says,
"The system is remarkably stable because it is made of limestone that is covered by a layer of sandstone. "
So, the only option to answer the question"What evidence does the author use to support the idea that the cave system is stable" would be C, It is made of limestone that is covered by a layer of sandstone.
Let me know if I'm correct!