IGCSE Biology Paper-1: Specimen Questions with Answers 181 - 181 of 208

Question 181

Edit

Question

MCQ▾

Mark the correct statement

Choices

Choice (4)
a.Global warming is because of ozone depletion
b.When organic waste enters a water body its BOD increases
c.The major contribution in greenhouse gases is of CH4
d.Question does not provide sufficient data or is vague

Answer

b.

Explanation

  • The earth surface re-emits heat in the form of long wave infra- red radiation, but a major fraction of it is absorbed by atmospheric gases (CO2, CH4), and does not escape into space.
  • The carbon-di-oxide and methane are the main greenhouse gases.
  • The relative contribution of various greenhouse gases to global warming is

  • The organic waste primarily contains biodegradable organic matter which is decomposed
  • by bacteria and other micro-organisms. The amount of such organic matter is measured by BOD (Biochemical Oxygen Demand) method. These organisms rob the dissolved oxygen of water due to which the dissolved oxygen content is sharply declined in downstream. This depletion of oxygen is responsible for the mortality of planktons, molluscs, fishes and other sensitive organisms (aquatic fauna). However, some tolerant species, like annelid worm, Tubifex and insect larvae (Chironomus- larvae) may survive in highly polluted and low DO water. Such species are known as ‘Indicator species’ for polluted water.