Before advertising on the second-hand market, many owners invest in repairing, painting, and polishing their cars as it increases the vehicle's retail value.

a) repairing, painting, and polishing their cars as it
b) repairs, painting, and polishing their cars as doing so
c) repairing, painting, and polish their cars as doing so
d) repairing, painting, and polishing their cars because it
e) repairing, painting, and polishing their cars because to do so

OA is E
Is there a special Concord Rule to pick an answer here? An Expert view will be appreciated.