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The figure shows a circle with radius r and diameter d marked.  Which equation is TRUE?
Ad = r
Bd = 2r
Cr = 2d
Dd = πr
Answer & Solution
Correct answer: B. d = 2r
The diameter d of a circle is twice the radius r — it's the longest chord, passing through the centre. So d = 2r. Hence r = d/2. The circumference C = 2πr = πd. Figure: Wikimedia Commons (CC) — alternative to NCERT diagrams.
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