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 .

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