Answer:
C. Downward force from rocks in the crust
Explanation:
Diamonds are different forms of element carbon. Diamonds are formed in Mantle and they require large amount of temperature and pressure for their formation. These come up in higher layers due to volcanic eruptions. The pressure is caused by the downward force from the rocks in the crust. The pressure is not constant everywhere because the thickness of the layer varies. Thus, diamonds are not that common.