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Figure shows a meter bridge, wire \( A C \) has uniform crosssection. The length of wire \( A C \) is \( 100 \mathrm{~cm} . X \) is a standard resistor of \( 4 \Omega \) and \( Y \) is a coil. When \( Y \) is immersed in melting ice the null point is at \( 40 \mathrm{~cm} \) from point \( A \). When the coil \( Y \) is heated to \( 100{ }^{\circ} \mathrm{C} \), a \( 78 \Omega \) resistor has to be connected in parallel with \( Y \) in order to keep the bridge balanced at the same point.
Temperature coefficient of resistance of the coil is
(a) \( 6.3 \times 10^{-4} \mathrm{~K}^{-1} \)
(b) \( 4.3 \times 10^{-4} \mathrm{~K}^{-1} \)
(c) \( 8.3 \times 10^{-4} \mathrm{~K}^{-1} \)
(d) \( 2.3 \times 10^{-4} \mathrm{~K}^{-1} \)
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