ON A TEST
Viral infections are difficult to treat medically because: (a) viruses do not respond to treatment with antibiotics (b) viruses have smaller ribosomes than the host cell (c) the symptoms of infection are always of short duration (d) viruses never elicit a response of antibody attack by the body's immune system

(a) viruses do not respond to treatment with antibiotics

you didn't give any options but heres what i can tell you: an inference is likely to change based on additional observations. the latter is something that you have seen or sensed using your sensory organs while the former is like a conclusion out from what you have observed.