13) E

1 – The fact that the outer circle produced by the antibiotics Q and R are very similar in size means that they could be equally as effective. 

2 – Bacterial resistance is not mentioned in the question and is thus irrelevant. The fact of the matter is that the antibiotics are doing their job which means that there is hardly any resistance. 

3 – S represents the distance up to which the bacteria are killed. This means that is statement could be correct.