Here's a problem to solve: A road cut shows a layer of shale on top of a layer of limestone. This indicates

Among the 4 following options, which is the correct answer?

- A: the shale layer is older than the limestone layer.
 
- B: a fault occurred during the deposition of the layers.
 
- C: the environment changed between the times of deposition.
 
- D: volcanic activity occurred when these layers were deposited.
 