What causes metamorphic rocks to form from existing rocks?

The rocks formed by the modification of other preexisting rocks in the interior of the Earth through a process called metamorphism. Through heat, pressure and / or chemically active fluids, the transformation of rocks undergoing structural and mineralogical adjustments takes place. The agents of metamorphism make possible that igneous rocks, sedimentary rocks or other metamorphic rocks, when subjected to pressures ranging from less than 1,000 to up to 16,000 bar, at temperatures ranging from 200 to 1,000 ° C, and / or an active fluid, cause changes in the composition of the same, providing new substances to them. The rock that is generated will depend on the composition and texture of the original rock, the time it was subjected to the effects of the so-called metamorphic process, as well as the agents of the same metamorphism. The precursor of a metamorphic rock is called protolite.

answer; /// i believe the correct answer is (thrombocytopenia); (a)

the answer is trees if i'm not mistaken