Which is the best summary of the passage?
The Castillo de San Marcos fort was built from coquina stone, a resource from the local environment. The air pockets in the stone, created as the sedimentary rock formed from clam shells, protected the fort from cannon fire during a British attack.
Coquina clams are responsible for the strength of the limestone that is commonly found in Florida. As the clams’ shells layer together over centuries, countless air pockets form, creating a stone that is very strong.
Spanish soldiers built the walls of the Castillo de San Marcos so thick—up to nineteen feet in places—that cannonballs had no chance of hurting the fort. The British attack had been expected for ten years, and the fort’s soldiers completed its construction just in time.
The story of the Castillo de San Marcos proves how natural resources like coquina stone are superior to man-made materials. Spanish soldiers were clever to harvest the abundant limestone and use it to strengthen their otherwise vulnerable fort.

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