The most suitable option for the first question is definitely <span>stratus clouds which usually stratch over the sky creating overcasts.
The next one should be </span><span>nimbus, as they dark and usually help to predict the storm.
And the last one </span>cumulonimbus that point on thunderstorms and so on.
Darwin's idea of the survival of the fittest means that certain organisms are more likely to survive with they traits in particular environments. This is because some organisms are more fit to live in a given environment whether it be due to temperature, weather, or predators.
One example is that dark moths are more fit for survival than light moths in many areas because they are able to camouflage against dark tree bark.
Hope this helps.
Alarge vacuoles cell walls and chloroplasts