Which of the following is true of all greenhouse gases?
a. they are naturally occurring.
b. they exist in fixed quantities.
c. they trap energy in the atmosphere.
d. they reflect incoming solar radiation.

Which of the following is true of all greenhouse gases?

C. THEY TRAP ENERGY IN THE ATMOSPHERE.

They are not naturally occurring. They are the result of human activity like using electricity, burning fuel, etc. 

They don't exist in fixed quantities and they don't reflect incoming radiation. The volume of greenhouse gases emitted depends on the activity done. It also does not stay in one place, it moves around and combines with other green house gases; trapping heat in the atmosphere and making the Earth warmer.

This type of galaxy is an approximately ellipsoidal shape and a smooth, nearly featureless image. they are the largest single star systems, in the universe and are built by the collisions of many smaller galaxies.