Roshni traveled at the speed of 66 km/h while going from point A to point B and returned via the same route at the speed of 92.4 km/h. What was Roshni's overall average speed during the two trips?

a) 79.2 km/h
b) 79.8 km/h
c) 80.1 km/h
d) 81.2 km/h