Newton's first law of motion, or the law of inertia, states: "An object will continue in its state of motion
unless acted upon by a net outside force."

Alex was traveling to school when he had a little accident. First, Alex's skateboard got caught on a crack in the sidewalk, but Alex himself kept moving forward. Then, a startled passerby stopped directly in front of Alex. Finally, Alex collided with the passerby, coming to a rather painful stop. At which point was Alex himself acted on by a net outside force?
A.
when he collided with the passerby
B.
when the passerby stopped
C.
when the skateboard got caught on the sidewalk
D.
when he kept moving forward