This Image of the Valles Caldera in New Mexico Shows

This image of the Valles Caldera in New Mexico shows a landscape in which there is a large flat meadow surrounded by rocky rims of volcanic rock. The meadow is several miles across. Why is this called a caldera?

A) It has volcanic rock.
B) It is a crater.
C) It is several miles across.
D) All of the above are correct.