What is the phi-phenomenon?

a. A region surrounding a magnet or electric current influencing other objects or fields without actually coming into physical contact with them.
b. The movement of a spot, created by the brain, between the two stimuli that occurs when the stimuli are presented nearly simultaneously
c. The phenomenon where a constant sound emanating from a moving object changes its pitch as it approaches or recedes from a stationary observer
d. The illusion created in a visual stimulus when an abrupt change in color or brightness creates illusory light or dark lines next to each band

B