According to the Rescorla-Wagner Model,why Does Blocking Occur

According to the Rescorla-Wagner model,why does blocking occur?

A) the initial conditioned stimulus requires most of the conditioning supported by the unconditioned stimulus leaving little to conditioning for the additional conditioned stimulus
B) when conditioning to the original conditioned stimulus makes it difficult for the animal to attend to the additional conditioned stimulus
C) when the initial conditioned stimulus is more salient than the added conditioned stimulus
D) the initial conditioned stimulus is contiguous with the unconditioned stimulus,while the added conditioned stimulus is not