Which sentence is worded correctly and avoids using a run-on?
A. The car had to swerve suddenly when they saw another car going through a red light; thankfully there was no accident.
B. The car had to swerve suddenly they saw another car going through a red light, thankfully there was no accident.
C. The car had to swerve suddenly when they saw another car, going through a red light thankfully there was no accident.
D. The car had to swerve suddenly, when they saw another car going through a red light, thankfully there was no accident.