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
(A) 42 (B) 59 (C) 87 (D) 90 (E) 100
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