Cause and Effect

On the Cause and Effect website, the most helpful part for me was the exercises that used the example about the people getting sick at the company picnic. With my luck, I was able to get the first two out of three answers correct, but for some reason unbeknownst to me, I couldn’t answer the last question correctly. It took me all of four tries to get the correct answer which slightly hindered me from understanding what the textbook style definition of Cause and Effect really was. The question was:
Five people became sick at the company picnic. Which of the following might be considered the most significant factor in determining the likelihood that the potato salad they all ate was the cause of their illness?
A. At least one other person ate the same potato salad and did not get sick.
B. The potato salad was supposed to be stored in the refrigerator, but it was left out the night before the picnic.
C. They ate varying amounts of potato salad.
D. They got sick right after eating the potato salad, but the incubation period for the illness was three days.
The correct answer was ‘D’, but I still don’t understand why this is. Hopefully I will get a better understanding before finals. :)