The Phage $\phi$ X174 Can Tolerate Mutations in the Non-Structural Region of Its

The phage $\phi$ X174 can tolerate mutations in the non-structural region of its genome more easily than in the structural region of the genome.Which of the following explains why?

A) Not all of the proteins coded in this region are packaged into the mature capsid.
B) Single-stranded DNA is not as easily mutated as double-stranded DNA.
C) Many of the non-structural proteins have more than one function.
D) Some of the non-structural proteins are non-essential for virus replication.
E) The presence of overlapping reading frames in this region of the genome.