Why Is a Viral Infection Difficult to Control Via Biochemical

Why is a viral infection difficult to control via biochemical means without harming the host?

A) Because they are very small.
B) Because their genomes can be so variable.
C) Because they recombine so readily, they often contain sequences homologous with their hosts. Therefore if these sequences are targeted, the host is also damaged.
D) Because viruses employ their hosts biochemical systems to reproduce.
E) Because they do not respond to antibiotics.