Which of the following adds/add carbon dioxide to the carbon cycle on the planet Earth? (1). Volcanic action (2). Respiration (3). Photosynthesis (4). Decay of organic matter Select the correct answer using the code given below.

Carbon dioxide enters the atmosphere whenever carbon stored in living matter, dead matter or rocks is oxidised or released. Photosynthesis does the opposite: it pulls CO2 out of the air and fixes it into plant biomass.

• 1. Volcanic action is correct. Magma degassing releases CO2 from the mantle and crust, a slow geological source.
• 2. Respiration is correct. Plants, animals and microbes oxidise glucose and release CO2 as a by-product.
• 3. Photosynthesis is wrong. It is a sink, removing CO2 and releasing oxygen, so it reduces atmospheric CO2.
• 4. Decay of organic matter is correct. Decomposers break down dead material and release CO2.

Core concept: the carbon cycle is a balance between sources and sinks. This links directly to Mains GS-III on climate change, carbon sinks and the role of forests. Note that fossil fuel burning is now the dominant human source, which this 2014 question does not test.

Trap: students see photosynthesis as the 'carbon process' and reflexively mark it as adding carbon. Ask which direction the gas moves.