Final answer:
Plastic flow occurs due to basal slip.
Step-by-step explanation:
The correct answer is B. Basal slip.
Plastic flow occurs in materials when they are subjected to stress beyond their elastic limit. This means the material is permanently deformed. Basal slip is one of the main mechanisms that can cause plastic flow. It involves the movement of dislocations within the crystal lattice of the material.
Here's why the other options are not correct:
A. High pressure: High pressure can indeed contribute to plastic flow by increasing the stress on the material. However, it is not the only or most direct cause.
C. Crevasses: Crevasses are cracks in the surface of a material. While they can be related to plastic flow, they are not the cause of it.
D. Low pressure: Low pressure typically has the opposite effect of promoting plastic flow. It reduces the stress on the material, making it less likely to deform permanently.
Therefore, the most accurate answer to the question "What causes plastic flow to occur?" is B. Basal slip.