Algae first produced oxygen about 2.7 billion years ago, but animals did not evolve until about one billion years later. Which statement explains this delay?
Plants had to evolve before animals could use them as a source of food
The percentage of oxygen had to rise high enough for animals to respire
The percentage of carbon dioxide had to fall low enough for algae to die
The oceans had to form before animals could live and reproduce in water
40 exam-style questions on AQA GCSE Chemistry 9.1 The composition and evolution of the Earth's atmosphere, covering 9.1.1 The proportions of different gases in the atmosphere, 9.1.2 The Earth's early atmosphere, 9.1.3 How oxygen increased, and 9.1.4 How carbon dioxide decreased. Each one has a worked solution and a mark scheme showing where the marks go.