Which of the Following Best Explains Why Angular, Crack- Bounded

Which of the following best explains why angular, crack- bounded, granitic blocks weather to spheroidal boulders?

A) Crack surfaces of blocks heat and cool more rapidly than edges and corners.
B) Interiors of blocks are constantly damp or wet because sunlight cannot penetrate.
C) Weathering agents have more direct access to edges and corners than to other parts of the block.
D) They slowly roll over in place and wear off the corners first.