Measuring the amount of deuterium in the universe allows us to set a limit on

the density of ordinary (baryonic) matter in the universe

Explanation:

 Deuterium detection is of interest because the amount of it may be related to the amount of dark matter in the universe, but precise measurements have been difficult to obtain. Due to the way in which deuterium was created in the Big Bang, an exact measurement of the amount of deuterium would allow scientists to set limits on the models of the great explosion.

Also, an exact measure of deuterium would be an indicator of the cosmic density of barions (ordinary matter), and that density of barions would indicate whether ordinary matter is dark and is found in regions such as black holes, gas clouds or brown dwarfs, or it is bright and can be found in the stars. This information will help scientists who try to understand the very beginning of our universe.

i think the answer is b

answer;

d. are made mostly of liquid

explanation;

-the general   composition and structure planets includes; they have a thick gas and liquid layer covering a small,solid core. mostly the gases are hydrogen and helium atmosphere and layers of liquid hydrogen. for instance neptune and uranus are examples of the outer planets, they have a core covered by ice and ammonia.

-neptune's atmosphere is hydrogen and helium with a bit of methane so there is no oxygen to breath and the interior is made up of frozen water and ammonia with a rock and iron core. thus, there is no liquid water or solid surface to survive.