"What would railroads do if every state had different requirements for smokestacks or track width?"

The train will move on the roads.

Explanation:

If every state had different requirements for smokestacks or track width, the trains should be run on the roads because if there is different requirement for track width, it is impossible to run same type of train on these tracks. One another act will be done and that was to make different types of trains for each state so that they can move on the tracks. But it is impossible for having different track width, it must be of same width.

b. the carbon cycle ensures living things have a source of hydrogen