Question

read the paragraph, then choose the word that best completes the sentence. the word biosphere refers to the mantle of life that covers earth. this mantle includes the air, soil, and water that sustain life on earth. it extends from the deepest ocean floor and to six miles (16 kilometers) into the atmosphere. however, new organisms have been discovered, and the definition of biosphere has been expanded to include them. these organisms are called extremophile organisms, and they may live deep within the earth, around scalding vents on the ocean floor, under the ice caps, and in harsh chemicals that were previously thought to be uninhabitable. extremophiles were once ________. a offensive b unnoticed c vulnerable d toxic

Explanation:

The paragraph mentions that new organisms (extremophiles) have been discovered. This implies they were not known or noticed before. "Unnoticed" best fits the context as it relates to the fact that they were previously unknown to us.

Answer:

B. unnoticed