During an el nino, warm water piles off of south america and cold water can't upwell. as a result, the can't rise and so small organisms have no food source.

The correct answer is - nutrients.

The cold water is very rich in nutrients, and those nutrients are crucial for the survival of the small organisms as they are their primary food source. When El Nino occurs though, the water is becoming warmer and stops the up-welling of  the cold water, which also means that the nutrients remain deep with it. As a result, the small organisms are left without their primary food sources, thus they struggle to survive, and even die out in the region on a large scale.

thymine. in rna, thymine is replace by uracil

humans can cause extinction of a species through overharvesting, pollution, habitat destruction, introduction of invasive species (such as new predators and food competitors), overhunting, and other influences. explosive, unsustainable human population growth is an essential cause of the extinction crisis.

explanation:

i didn't have the answer choices , but i tried my best.