According to Pavlov,what is necessary for classical conditioning to occur

A)  the unconditioned and conditioned stimuli must occur together in time and space 
B)  the use of sign-stimuli to prevent resistance to extinction 
C)  a very strong unconditioned stimulus 
D)  a very intense conditioned stimulus

According to Pavlov, classical conditioning occurs when the unconditioned stimulus (UCS) and the conditioned stimulus (CS) are presented together in time and space. This allows for the association between the two stimuli to form, resulting in a learned response to the conditioned stimulus. The use of sign-stimuli (B) may be helpful in some cases, but it is not necessary for conditioning to occur. Similarly, a very strong unconditioned stimulus (C) or a very intense conditioned stimulus (D) may facilitate conditioning, but they are not required.