What is differential hebbian learning?

• A. synaptic strength is proportional to correlation between firing of post & presynaptic neuron
• B. synaptic strength is proportional to correlation between firing of postsynaptic neuron only
• C. synaptic strength is proportional to correlation between firing of presynaptic neuron only
• D. synaptic strength is proportional to changes in correlation between firing of post & presynaptic neuron