A crack must be present on a rock that undergoes frost wedging.
The correct answer is "a crack". Frost wedging is a mechanical weathering process that occurs when water seeps into cracks or pores within a rock and freezes. When water freezes, it expands and exerts pressure on the surrounding rock, causing the crack to widen and ultimately leading to the fragmentation of the rock. So, in order for frost wedging to occur, a crack must be present on the rock.
