Which must be true for metamorphism to occur?

a. the parent rock must be in solid form.
b. the parent rock must be igneous.
c. the grain size must become smaller.
d. the process must take place underwater.

I think the correct answer from the choices listed above is option A. For metamorphism to occur, the parent rock must be in solid form. Metamorphism is the alteration of the structure of a rock by factors like heat and temperature so it must be the material should be in solid form.

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