Study the short passage below, read the question, and select the correct answer. Passage:
The little boy had a policeman for a friend. He acquired him out of a clear sky. He ran out of the schoolyard to go home for his noon lunch, tripped over a rough spot on the sidewalk, and fell so hard and so flat that for gasping moments he could not draw a breath. The police
man happened to be passing by. Robert felt himself being lifted and pounded on the back. The first breath that came was agony and wond
er, for drawing it had seemed impossible. It was only with the third that he realized his knees were hurting, and he looked down to see the
m tom and bleeding. He became aware of the policeman and then it was unthinkable to cry.

Why did the boy fall down?

A) His friends pushed him

B) He was running from the police

C) He was running home

D) He tripped over a rough spot