The main agents that cause metamorphic rocks to form are
A. weathering and erosion.
B. heat and pressure.
C. melting and cooling.
D. deposition and compaction.

B. Heat and Pressure

Explanation:

This is because the rocks were once igneous or sedimentary which underwent change through physical and chemical processes...

I hope u will understand it :-):-)

iii. and iv. only is the right answer