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Biology · 2026 · 2 marks
CBSE 2026 · Region 4 · Set 1 · Q19
A laboratory assistant collected three water samples, namely river water, untreated sewage water and secondary effluent discharged from a sewage…
A laboratory assistant collected three water samples, namely river water, untreated sewage water and secondary effluent discharged from a sewage treatment plant. But the laboratory attendant did not note which was which. The samples were labelled A, B and C and subjected to a BOD test. The BOD values of the three samples A, B and C were recorded as $\displaystyle 30 \mathrm{mg} / \mathrm{L}, 8 \mathrm{mg} / \mathrm{L}$ and $\displaystyle 400 \mathrm{mg} / \mathrm{L}$ respectively.
(a)
Which sample of the water is most polluted ?
(b)
Can you assign the correct label to each, assuming the river water is relatively clean ?
Marking-scheme solution
(a)
Sample C is most polluted (Highest BOD)
(b)
Sample A – River water
Sample B – Secondary effluent (Least BOD)
Sample C – Untreated sewage water (Highest BOD)
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CBSE Class 12 Biology past-paper question from the 2026 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.