the difference between join in and take part in

[san2612]

1. The rumors go that Jason will be arrested. He is said _________ a bloody robbery
A to have taken part in B to have joined in
C to join in D to take part in

I chose B but the answer is A. WHAT'S THE DIFFERENCE?

 

[riquecohen]

The Macmillan Dictionary tells us that to join in means to become involved in an activity that others are already doing. To take part in implies active involvement or participation (from the start.) The difference is fairly subtle. I agree that A is the better answer.