There is a theory that gravity could pull galaxies back together, causing a reverse big bang. Which observation makes this theory less likely than the idea that the universe may continue to expand?

well

explanation:

answer by yourhope:

answer is

human impact on the environment can be seen in the deterioration of the !

: )

hydrogen is a compound or a single element and a element is a pure substence that cannot be separated into simpler subtances

clorine is a compound its a chemical substance that forms when two or more different elements combine

so both hydrogen and clorine are compounds