Final answer:
Heavy rainfall makes clay slippery due to its fine, plastic particles and turns sandstone unstable as the water weakens the bonds between its grains, leading to potential landslides or mudflows. correct option is A.
Step-by-step explanation:
Heavy rainfall can cause clay to lubricate and the sandstone to become unstable. When the water saturates the clay, the fines particles that give clay its plasticity and cohesion lead to a reduction in friction among the particles, making it slippery. As for sandstone, which typically contains quartz that is resistant to abrasion, the presence of excess water can cause the bonds between the grains to weaken. Combined with the weight of the water, this can lead to a reduction in the internal strength of the sandstone, rendering it unstable. This instability is particularly hazardous on slopes, where it can lead to landslides or mudflows, carrying debris along the way and potentially causing extensive damage and flooding.