How would you ensure economic fuel consumption?

1. A By shifting to the appropriate gear and maintaining a moderate driving speed of up to 90 k.p.h.
2. B By driving in low gear and maintaining a driving speed of more than 120 k.p.h, without any aggressive accelerations or decelerations.
3. C By shifting as much as possible to neutral gear while driving downhill.
4. D By driving in low gear and maintaining a driving speed of more than 90 k.p.h.