It is difficult to make antiviral drugs becausea. Viruses use the host cell's machinery to grow and divide.b. Viruses are surrounded by protein coats.c. Viruses are one of the borderlines of living and non-living states.d. Viruses are very small in size.

The most relevant reason for why it's difficult to develop antiviral drugs is:

a. Viruses use the host cell's machinery to grow and divide.

This is a fundamental challenge in combating viruses. Unlike bacteria, which are independent living organisms, viruses require host cells to replicate. They hijack the cellular machinery of the host cell to produce more viruses, making it challenging to target the virus without harming the host cell. This complexity in the viral replication process presents obstacles in developing drugs that effectively target viruses while minimizing harm to the host organism. While the other options (b, c, and d) describe characteristics of viruses, they do not directly address the difficulty in developing antiviral drugs.