Answer:
The cost of modifying tanks and engines in cars, changing fuel stations,
Explanation:
Natural gas has a higher cost because cars will need to have their engines and tanks modified to be able to use natural gas or people will have to buy a new one. Also, fuel stations will need to be changed or new ones will have o be built which is a expensive infrastructure.
The answer is; movement toward or away from Earth
Stars that move closer to the observer will be percieved as blue in the electromagnetic spectrum while starts moving away from the observer will appear red and orange. This is what is called the Doppler shift effect. This phenomenon is similar to the higher pitch of the siren of an ambulance approaching you and the low pitch of the siren when the ambulance passes you and is moving away from you.