Put the letter of your choice in the blank.  Use lower case…

Put the letter of your choice in the blank.  Use lower case letters as indicated in the options.  A simple spinal reflex goes along which of the following reflex arcs [1] a. effector, afferent neuron, integration center, efferent neuron, receptor b. receptor, afferent neuron, integration center, efferent neuron, effector c. effector, efferent neuron, integration center, afferent neuron, receptor d. receptor, efferent neuron, integration center, afferent neuron, effector