Which event led to United States involvement in World War II?
A. The Japanese attacked Pearl Harbor.
B. The Axis powers waged war.
C. The Allies invaded Normandy.
D. U.S. allies were being attacked.
No sir, World War II was caused when Japanese air planes bombed Peal Harbor!
D was already happening -- in Europe and elsewhere. However, read the question carefully. I notice that most of the time, when students guess, they've either not read their assigned materials or they haven't read the question accurately.
The question asks "which event" (singular). That's why the answer is A.