Two students each prepared an agar plate from the same sample of bacteria. The Figure shows their two plates after incubation.
Figure: The agar plates prepared by student A and student B after incubation

Give the letter of the plate that has become contaminated. Give one piece of evidence from the Figure to support your answer.
Suggest two mistakes in aseptic technique that could have caused this contamination.
The students want to use their plates to compare the effect of two antibiotics on this bacterium. Explain why contamination is a problem for this comparison.
163 exam-style questions on AQA GCSE Biology 1.1 Cell structure, covering 1.1.1 Eukaryotes and prokaryotes, 1.1.2 Animal and plant cells, 1.1.3 Cell specialisation, 1.1.4 Cell differentiation, 1.1.5 Microscopy, and 1.1.6 Culturing microorganisms. Each one has a worked solution and a mark scheme showing where the marks go.