Given below are two statements: Statement I: Long stretches of major rivers of…
2020
Given below are two statements:
Statement I: Long stretches of major rivers of India have very low Biochemical Oxygen Demand (BOD) values.
Statement II: In rivers oxygen is added into water by diffusion from air due to its turbulent and rapid flow.
In the light of the above statements, choose the most appropriate answer from the options given below:
- A.
Both Statement I and Statement II are correct
- B.
Both Statement I and Statement II are incorrect
- C.
Statement I is correct but Statement II is incorrect
- D.
Statement I is incorrect but Statement II is correct
Attempted by 1 students.
Show answer & explanation
Correct answer: A
Concept
Biochemical Oxygen Demand (BOD) measures the amount of dissolved oxygen that micro-organisms need to break down the organic matter in a water sample. A LOW BOD therefore means little decomposable organic pollution and good water quality; a HIGH BOD signals heavy organic pollution. Separately, the dissolved oxygen in a river is continually replenished by re-aeration: oxygen diffuses from the atmosphere into the water, and this transfer is fastest where the flow is fast and turbulent, because turbulence keeps exposing fresh water to the air at the surface.
Applying it to the two statements
Statement I — Across long, free-flowing stretches of India's major rivers, monitored BOD generally stays low (within the desirable range), so these reaches do show very low BOD values. The statement is correct.
Statement II — Turbulent, rapidly flowing water mixes air into the stream and exposes more surface, so oxygen is indeed added to the water by diffusion from the air. This re-aeration mechanism is correctly described. The statement is correct.
Cross-check
The two statements are also linked by cause and effect: the same vigorous, turbulent flow that drives oxygen into the water (Statement II) is what keeps these long stretches well-oxygenated, which in turn keeps organic-matter breakdown efficient and BOD low (Statement I). Each statement is independently true, and they reinforce each other.
Result: Both Statement I and Statement II are correct.