What Is the "Phi" Phenomenon

Multiple Choice

 What is the "phi" phenomenon?

A) When one stationary object alternates with another, we see movement.
B) People who know each other well are likely to think the same thing at the same time.
C) The more times one has been depressed, the easier it is to become depressed again.
D) A sound heard at the same time as a visual stimulus seems to come from that object.