What Event Prompted a Massive Wave of "Sit-Ins" Across the Country

What event prompted a massive wave of "sit-ins" across the country?

A) African Americans went on a hunger strike after being refused service at a diner.
B) An African American soldier refused to stand and salute an abusive white officer.
C) A white restaurant owner assaulted a black customer when she asked to be served in a white area.
D) Three African American college students refused to leave a lunch counter after being denied service.
E) Three African American women refused to get off a bus after being asked to move to the back.