They must occur in the DNA, which is in each cells nucleus. More specifically the nucleolus of the cell. A mutation occurs in an allele, where a base triplet (or a codon) gets transcribed incorrectly.
Answer:
B. Viruses are smaller than Bacteria and Archaea
C. Viruses do not belong to any of the three domains: Bacteria, Archaea, and Eukarya
E. Viruses are as old as Bacteria and Archaea
F. Viruses are unable to perform replication on their own
Human Immunodeficiency Virus -- this is HIV, the virus that causes AIDS.
Flu viruses. There are lots of different strains of flu virus, which is why new vaccines have to be made every year.
Cold virus. The common cold is caused by a virus.