Identify a nitrogen base from a feature of its structure

This type of question, asked 3 times: 16 May 2023 S2, 22 Apr 2025 S1, 15 May 2026 S1. All 3 questions below, oldest first.

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Question 1
16 May 2023 S2
99 $Which from following nitrogen bases of nucleic acids is derived from purine?$
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Question 2
22 Apr 2025 S1
52 $-NH_{2}$
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Question 3
15 May 2026 S1
46 $Which of the following nitrogen bases in nucleic acids contains two keto groups?$
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