earth's early atmosphere lost its hydrogen and helium gases because the

a.

gases were too light to be held by gravity

b.

gases condensed, forming oceans, rivers, and lakes

c.

planet was so hot that all gasses escaped into space

d.

gases were replaced by carbon dioxide from volcanic eruptions

A & C

Explanation:

The combination of the earth's weak gravity and its closeness to the sun does not allow it to hold hydrogen and helium gases in its atmosphere. Its relative closeness to the sun means it is hot enough such that the helium and hydrogen molecules would have high kinetic energy. Remember that gravity acts strongly on larger masses, therefore it would require very strong gravity to have an influence on lighter gas molecules like hydrogen and helium let alone when they have a high kinetic energy. This means these molecules can easily escape the earth’s atmosphere into space.

Planets that are larger (meaning they have a stronger gravity) and farther from the sun (meaning these molecules won't have a very high kinetic energy) are able to hold these lighter gases in their atmosphere. Examples of such planets are Jupiter.

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Combination of C and A.

Explanation:  As the gases are heated, the velocity of the gases will increase, now, there is something called "escape velocity".

This "escape velocity" is the velocity needed for an object to escape the planet gravity. This phenomenon, where gases are heated and they scape into space, is called a hydrodynamic escape.

There is also Jean's escape, which actually describes how single particles reach the escape velocity.

Then, you can see that both things have an impact, if the gravity is low enough, the escape velocity will be less, and the particles will need less velocity to escape.

If the temperature is big enough, the velocity of the particles will be big enough to escape from the atmosphere.

In our particular case, option C has a bigger impact, because we first need to have an increase in temperature in order to see the effects of the option A.

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