If a whole number is multiplied by itself, the ones\textquotesingle{} digit of the product could be $$1 (1\times1)$$ or $$5 (5\times5)$$ or $$9 (3\times3)$$, but not $$7$$. 
If a whole number is multiplied by itself, the ones\textquotesingle{} digit of the product \emph{cannot} be． 
Options:
a) $$1$$ 
b) $$5$$ 
c) $$7$$ 
d) $$9$$ 