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A sample of pure Cu (3.18g ) heated in a stream of oxygen for some time gains in mass with the formation of black oxide of copper (CuO). The final mas is 3.92 g. What per cent  of copper remains unoxidised?

(a)$\approx$6.5

(b)$\approx$6.9

(c) $\approx$7.6

(d) $\approx$7.9