These are his friends. is that an observation or an inference?

If he hasn't told you that these people are his friends, then it's an inference when you see them.

a) electrical > light, sound, and thermal< < < <

) the mechanical energy of the firefighter moving.< < < < a) electrical > light, sound, and thermal< < < <

c) both a and b.

1/0.14nm = 10^10/1.4 a lot