The following image is a picture of the Veil Nebula made by combining several photos taken by the Hubble orbiting telescope.

The colors in the picture indicate the different elements present: blue is oxygen, green is sulfur, and red is hydrogen.

How do astronomers know which element is present where?
A. by comparing the density of the gas to the densities of various elements

B. by measuring the temperature of each part of the gas

C. by looking for specific wavelengths of light that each element produces

D. by observing chemical reactions taking place in the nebula