Is someone walking up to you from behind? A friend is walking up to you from behind. What three observations would help you predict when they would reach you?

Three things i will observe are :

The sound of their movement when approaching meThe frequency of their foot stepThe distance they are away from me

Explanation:

In my opinion, the three points highlighted above can go a long way to help predict how soon or when they would reach meThe sound of their foot steps will determine how far or close they are from from me, if the sound is loud they are closest, if it is low then they are at a distance father.The frequency of their foot steps tell me about the speed with which they are approaching me, if the frequency is high, then they will approach sooner and vice versaThe distance they are way from me can help estimate when they would reach me, though still dependent on items  one and two above.

A

Carbon dioxide is the correct answer