With reference to the planet Earth, consider the following statements:

Answer: (b) II only

Only II is right. Phytoplankton and photosynthetic bacteria make about half of Earth’s oxygen.

Statement by statement

1. ✗ WRONG — the ocean makes at least as much Marine phytoplankton, seaweeds and cyanobacteria make roughly half of Earth’s oxygen (some estimates go to 70–80%). All land plants together make the rest, and rainforests are only part of that land share. A mature rainforest also uses up almost all the oxygen it makes, through its own respiration and decay.

2. ✓ CORRECT — about half Tiny drifting algae (diatoms, dinoflagellates) and cyanobacteria do this. One cyanobacterium, Prochlorococcus, is the smallest photosynthetic organism known, and it alone may make up to ~20% of the biosphere’s oxygen.

3. ✗ WRONG — it is the other way round A litre of air holds about 210 mL of oxygen (21%). A litre of well-oxygenated water holds only about 6–7 mL (≈ 9 mg at 20 °C), roughly 30 times less. That is why fish need gills that push water over a huge surface, and why low-oxygen “dead zones” form so easily.

The idea

The oxygen in the air was built up over billions of years, first by cyanobacteria in the sea. Even today a large share of the yearly production comes from tiny floating organisms in the ocean, not from forests. And water holds very little oxygen compared with air, because oxygen dissolves poorly.

Sunlit ocean surface → Phytoplankton and cyanobacteria photosynthesise → About half of the oxygen made on Earth each year → Land plants make the other half

Why the others are wrong

(a) Statement I is wrong: rainforests do not out-produce the oceans.

(c) Both I and III are wrong. III has it reversed, since air holds far more oxygen than water.

(d) Statement II is correct, so “none” cannot be right.

The trap

“Amazon = lungs of the Earth” is the picture statement I plays on. Rainforests matter hugely for biodiversity and carbon storage, but in oxygen the ocean out-produces them.

Also know

Diatoms have silica walls, while cyanobacteria are bacteria that carry out photosynthesis. Zooplankton are drifting animals that feed on plankton. Nutrient pollution can cause algal blooms; when the algae die, microbes use oxygen to break them down. This is eutrophication, which can leave fish without enough oxygen.