eco-friendly

Script:

Mr Ha: Now, let's turn to future means of transport. How do you think people will travel in 2050, Tom?

Tom: I think people will use bullet trains. Bullet trains will be faster and safer than cars and they can help avoid traffic accidents.

Lan: I think skyTrans will also be popular, too. They will not use up much space and will be safe because they run on autopilot.

Tom: But skyTrans may be too expensive for students to use. Bamboo-copters will be cheaper and easier for them to use. They will just put on their bamboo-copters and fly to school.

Mr Ha: Sounds interesting. How about travelling on sea?

Lan: I think people will use solar-powered ships to travel on sea. They will be eco-friendly and comfortable...

A. alcohol B. comment C. chemical D. proceed