Sunila drove her car at an average speed of 45 km/h for the first 75 km of a trip, and then at an average speed of 70 km/h for the remaining 45 km of the trip. If she made no stops during the trip, what was Sunila's average speed (in km/h) for the entire trip?

A

52

B

$$51\frac{3}{7}$$

C

51

D

$$51\frac{93}{97}$$