nuclear fuel. biogas. coal gas. diesel fuel.
I gave my students this multiple choice question: In 1995, a private bus company in Washington, D.C., introduced a program to run all its tour buses with fuel obtained from soybeans. The buses are powered by

Only one answer is correct among these 4 choices:
- nuclear fuel.
- biogas.
- coal gas.
- diesel fuel.

Could you tell me which one is correct?

biogas.