18. When magma cools quickly, what kind of texture or crystals does a rock have? A. Coarse-grained or large crystals. B. Medium-grained or medium crystals. C. Fine-grained or small crystals. D. A mixture of all types of grains. E. A mixture of all types of crystals.

C. Fine-grained or small crystals.

Explanation:

When the rocks are rapidly cooled, and during rapid cooling, the fine grained of rocks are formed, as during the process of crystallization, the minerals does not get enough time to crystallize and forms a small and fine - grained crystals.

And,

In case of slow cooling, the rocks generally have, coarse grained.