Consider the following statements: Which of the statements given above is/are correct?

Answer: (c) 1 and 3 only

Magnetic reversals and life changing the atmosphere are right.

Statement by statement

1. ✓ CORRECT The magnetic field has reversed many times, every few hundred thousand years on average.

2. ✗ WRONG The early atmosphere had almost no free oxygen and lots of CO₂.

3. ✓ CORRECT Photosynthesising life (cyanobacteria) filled the air with oxygen.

The idea

Earth’s air has changed as gases escaped from its interior and living organisms altered their surroundings. Free oxygen became abundant only after oxygen-producing photosynthesis developed. The magnetic field is produced by motion in the liquid outer core, a separate process. Its reversals are irregular events, not a fixed timetable.

Volcanic gases contribute to early air → Earth cools and oceans form → Photosynthetic microbes release oxygen → Oxygen first reacts with minerals and other materials → Oxygen later accumulates in the atmosphere → An ozone shield can develop

Why the others are wrong

(a) Drops the role of life.

(b) Accepts 54% early oxygen.

(d) Accepts 54% early oxygen.

The trap

A precise figure (54%) makes a false claim sound researched.

Elimination technique

An oxygen-rich atmosphere at Earth’s formation conflicts with the later biological buildup of free oxygen. Reject statement 2, which removes (b) and (d); then retain the independently true role of living organisms to distinguish (c) from (a).

Also know

The Great Oxidation Event occurred roughly 2.4 billion years ago. Water vapour, carbon dioxide and nitrogen were important gases in early atmospheric development; their exact proportions are not securely known. Magnetic reversals are preserved in iron-bearing minerals as lava cools. Symmetrical magnetic stripes on the ocean floor support sea-floor spreading.