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 \free - 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
submit

Explanation:

The paragraph describes viruses as difficult to kill, which aligns with the meaning of "persistent" (continuing firmly or obstinately in a course of action despite difficulty or opposition). "Solitary" (alone) is not supported by the text. "Cooperative" (working together) is not relevant. "Protective" (serving to protect) is also not in line with the description of viruses as agents of destruction and hard - to - kill entities.

Answer:

A. persistent