An unknown igneous rock has very small crystals. the rock

a : must be granite
b : cooled slowly
c : cooled quickly
d : was formed from dissolved minerals

The rock cooled quickly.

Igneous rock are divided into two; Extrusive and intrusive. The types of igneous rocks that cool quickly and form very fine-grained crystals are called extrusive igneous rocks. Extrusive igneous rocks are formed when magma exits and cools very near or above the earth’s surface. The magma cools almost immediately when exposed to the relatively cool temperatures of the earth’s atmosphere.  

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