Correct answer: (b) 20 dB
Explanation
- A
10 dB
10 dB is a very faint sound (near the threshold of hearing in some scales). It is quieter than the keyed rustle. Do not under-state leaf noise as 10 dB here.
- B
20 dB
(b) Official key: Rustling of tree leaves under normal circumstances is keyed at 20 dB. Among 10, 20, 60 and 100 dB, the stored rustle is 20. Honour that official level.
- C
60 dB
60 dB is ordinary conversation or a busy office, far louder than a leaf rustle. It is not the key.
- D
100 dB
100 dB is near a motorcycle or loud machinery and can damage hearing. Leaves do not rustle at that intensity. 100 dB is not the answer.
Summary. Official key is (b). Normal leaf rustle in this paper is 20 dB. 10 dB is too faint; 60 dB is speech; 100 dB is hazardous noise. Keep 20 dB. The official letter is (b).
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