Which of the Following Situations Illustrates the Phi Effect

Which of the following situations illustrates the phi effect?

A) As a word is gradually masked so it becomes harder to see, the viewer easily reads it at one point, then suddenly cannot read it anymore.
B) Each cell in the visual cortex only receives a signal from part of one retina.
C) Researchers using an fMRI to monitor brain activity see brain areas activated when the participant views a flashing pattern.
D) A small line and longer line flash in alternating sequences, causing the viewer to think a single line is moving back and forth and growing longer and shorter.