Which is a run-on sentence?

A.
We thought we were the only ones on the tour, dozens of people were lined up at the gate.

B.
Her dog, Goldie, goes to agility training, which directs dogs through an obstacle course.

C.
Heather, who brought a tray of cakes and cookies, set them on the table at the potluck.

D.
When you come home from school, please put the trash out and water the lawn.