A student investigates the antimicrobial properties of two natural plant extracts, Garlic extract (G) and Tea tree oil (T), on the growth of Staphylococcus aureus bacteria on agar plates.
They place paper discs soaked in the extracts, along with a control disc (C) soaked in sterile water, onto an agar plate inoculated with the bacteria. After incubating the plate at 25 ∘C25\ ^\circ\text{C}25 ∘C for 24 hours, circular zones of inhibition (where bacterial growth is prevented) are observed around some of the discs.
State the physical measurement the student must take in order to calculate the surface area of the zone of inhibition.
If the measured diameter of the zone of inhibition for Garlic extract (G) is 3.4 cm, calculate the area of this zone. Give your answer to 2 significant figures. (Use π=3.1416\pi = 3.1416π=3.1416).
Describe how the student could check if the results of this investigation are repeatable.