Multi-select problem: How does the next paragraph end?

Several dirt bikers are standing under a garage behind a stand preparing to race. someone

Choices:
[a]. is let loose by a crowd and they take off on a dirt bike..
[b]. is costumed in paint and is riding on a dirt bike..
[c]. is then shown speaking and clips of a track like polo are shown..
[d]. calls a mark and they all stand up on their pedals preparing to be released..
[d].