Rules Quiz

[LDUB]

I just came across this quiz a few minutes ago. http://www.rulesofbaseball.com/quiz1.html

It is made by http://www.rulesofbaseball.com/, but I think they have a few answers wrong.

One question really stood out to me. In it F1 stumbles and ends up falling, he ends up lying on top of first base. The BR is running down the line. He steps on F1's back, because F1 is totaly covering the base. BR falls on the ground. F2 picks up the ball and tags BR. It said that BR is out on appeal for missing first. Now mabye I'm missing something, but how is that not obstruction?