Scientific researchers discovered agents that behaved like bacteria causing diseases such as rabies and hoof-and-mouth, but were much smaller. At this time it became the general view that viruses were biologically "alive." This 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 and the scientific view changed: viruses are complex biochemical mechanisms but not alive. When not in contact with a host, the virus remains dormant. During this time there are no biological activities occurring and the virus is nothing more than a static organic particle. In this state viruses can remain for extended periods of time, waiting to come into contact with the appropriate host. When in 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.

According to the passage all BUT ONE is a reason to classify viruses as living. That is they

A)
are capable of protein synthesis.

B)
contain a nucleic acid, DNA or RNA.

C)
response to changes in the environment.

D)
lack mechanisms for metabolic function.