The correct options are
B viruses are on the borderline of living and nonliving
C viruses use the host cell machinery to grow and divide
It is difficult to make anti-viral drugs because viruses exist as non-living outside a host cell and so they cannot be called as living or non-living. Hence, it is difficult to study them and find a cure against the infections caused by viruses. Viruses also have an ability to modify themselves frequently giving rise to different strains. Therefore, it is very difficult to make drugs against a particular virus.