A researcher investigated the antimicrobial effectiveness of two plant extracts, thyme oil and tea tree oil, against Staphylococcus aureus bacteria grown on agar in a Petri dish.
Filter paper discs soaked in each plant extract were placed on the agar. After incubation at 25 ∘C25\text{ }^\circ\text{C}25 ∘C for 24 hours, circular zones of inhibition (clear areas with no bacterial growth) appeared around the discs, as shown in the diagram below.
What physical measurement must the researcher take from the Petri dish in order to calculate the surface area of the zone of inhibition?
The researcher measures the diameter ddd of the zone of inhibition around the tea tree oil disc to be 33 mm33\text{ mm}33 mm. Calculate the area of this zone. Give your answer to 3 significant figures.
Describe how the researcher could check if these experimental results are repeatable.