Which of the following does not contain number of neutrons equal to that of \[\begin{array}{c}40\\18\end{array}Ar\]
a) \[\begin{array}{c}41\\19\end{array}K\]
b) \[\begin{array}{c}43\\21\end{array}SC\]
c) \[\begin{array}{c}40\\21\end{array}SC\]
d) \[\begin{array}{c}42\\20\end{array}Ca\]

Answer: c
Explanation: \[_{18}Ar^{40}\] contains 22 neutrons and \[_{21}Sc^{40}\] contains 19 neutrons. The number of neutrons = (A – Z)