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# Find the number of significant figures in the mass difference between a proton and a neutron. Given that mass of a proton $= 1.672649 \times 10^{27}$ mass of a neutron $= 1.674954 \times 10^{-27} kg$

$\begin{array}{1 1}(A)\;2.305 \times 10^{-30}\;kg \\(B)\;2.3 \times 10^{-3}\;kg \\(C)\;7.8 \times 10^{-7} \;kg \\(D)\;2.5 \times 10^{-9}\;kg \end{array}$

Mass difference $=(1.674954 -1.672649) \times 10^{-27}$
$\qquad= 0.002305 \times 10^{27}$
$\qquad= 2.305 \times 10^{-30}$
Hence A is the correct answer.