What is the fine-grained topic of the following text: Compare that to the 2.6 liters required by high-speed train, 6.6 liters by airplane and 7.6 liters by gas-powered car, and it's clear that the bus is a more environmentally-conscious option for your bus transportation from Banff to Lethbridge. Only return the topic.
Comparing the environmental impact of different modes of transportation from Banff to Lethbridge