Question

read the paragraph, then choose the word that best completes the sentence. found in all life forms, viruses are prolific agents of destruction. viruses are made of genetic material surrounded by protein. they are responsible for a great many illnesses and diseases. they are not considered true living, because in order for them to replicate, it is necessary to attach themselves to a living cell. quickly moving and often too small to detect, viruses are also difficult to kill. this is because viruses are able to change as they reproduce. scientists continue to research viruses in an attempt to find cures for the diseases that they cause. however, it is a challenge to develop medicines that target the virus and leave other important cells undamaged. viruses are ________ a persistent b solitary c cooperative d protective

Explanation:

The paragraph describes viruses as being difficult to kill, able to change as they reproduce, and constantly present in causing illnesses. The word "persistent" which means continuing firmly or obstinately in a course of action in spite of difficulty or opposition best fits this description. Viruses are not solitary as they need a host cell, not cooperative in a positive - sense, and not protective.

Answer:

A. persistent