It takes Sharon one hour to drive the 59\:km from her home to her office. Her drive includes 20 minutes on a highway and 40 minutes on city roads. If her average speed when she is on city roads is 45\:km/h, the average speed, in km/h, at which she drives on the highway is
Answer Choices
A. 42
B. 59
C. 87
D. 90
E. 100