Simon rents a car in the city to go skiing in the mountains 100 kilometres away. He stops halfway to pick up his girlfriend Nina at her house, and drives the remaining 50 kilometres with her. On their way back, Simon drops Nina at her house and drives back to the city alone. He returns the car and pays $50 for the rental and $10 for the gas. Simon and Nina share the expenses equitably. How much should Nina pay?

A：10
B：15
C：20
D：25
E：30