Scientific researchers discovered agents that behaved like bacteria causing diseases such as rabies and hoof-and-mouth, but were much smaller. It became the general view that viruses were biologically "alive." This perception changed in 1935 when the tobacco mosaic virus was crystallized and scientists demonstrated that the particles lacked any mechanisms necessary for metabolic function. It was determined that viruses consisted of a nucleic acid, DNA or RNA, surrounded by a protein shell. Viruses exist in two distinct states. When not in contact with a host cell, the virus remains entirely dormant. During this time there are no biological activities occurring and the virus is nothing more than a static organic particle. Viruses can remain like this for extended periods of time, waiting to come into contact with the appropriate host. When the virus comes into contact with a host, it becomes active, reacts to its environment and/or host and directs its efforts toward self-replication. The viral goal now is to produce more viruses to attack host cells.

The characteristics of life: many are listed in the passage describing viruses. One important piece of evidence arguing against life for viruses is implied by the passage; not stated directly. Viruses are not living, because, as implied by the passage, they

A. Lack a nucleus
B. Have no DNA
C. Are not cellular
D. Do not reproduce