These gases<span> may have consisted of hydrogen (H2), water vapor, methane (CH4) , and carbon oxides.
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The main reason is because they are heavy, and because they don't have to chase their food. They're not predators and their shells give them protection like running away gives protection to other herbivores.

The Correct matches are :
<u>Selective</u><u> </u><u>breeding</u> :
The process by which desired traits of certain Plants and Animals.
<u>Inbreeding</u> :
Crossing parent organisms with the same forms of a trait to produce.
<u>Hybridization</u> :
Crossing parent organisms with different forms of a trait to produce.
Scientific method
a method of procedure that has characterized natural science since the 17th century, consisting in systematic observation, measurement, and experiment, and the formulation, testing, and modification of hypotheses.