Final answer:
The crystals in igneous rocks that form from rapidly cooling magma are smaller due to the lack of time for crystal growth.
Step-by-step explanation:
The speed at which magma cools plays a critical role in determining the size of crystals in igneous rocks. When magma cools rapidly, as is the case with extrusive igneous rocks, there isn't enough time for large crystals to form. Instead, the rocks have a very fine-grained texture to the point where crystals can hardly be seen without magnification. This contrasts with intrusive igneous rocks, which cool slowly and develop coarse grains with large, visible crystals.
The correct answer to the student's question 'The faster a magma cools, the ________ the crystals will be in the resulting igneous rock' is: smaller. A fast cooling rate results in smaller crystals because there is insufficient time for them to grow.