To allow cyclists to position in front of other traffic
You should always stop at the first white line. Avoid going into the marked area which is reserved for cyclists only. However if you have crossed the first white line at the time the signal changes to red you must stop at the second line even if you are in the marked area.
B To let pedestrians cross when the lights change
C To prevent traffic from jumping the lights
D To let passengers get off a bus which is queuing