A delivery truck must travel from point P to point Q. Point Q is 6 km north of a straight east–west highway and 12 km east of P. The truck can drive on the highway at 60 km/h and can cross the open field at 40 km/h. If the driver leaves the highway after traveling x kilometers east from P and then heads straight across the field to Q, which value of x (to the nearest tenth of a kilometer) minimizes the time required for the trip?
a) 8.0 km
b) 6.6 km
c) 9.9 km
d) 4.0 km