If a whole number is multiplied by itself, the ones\textquotesingle{} digit of the product \emph{cannot} be． 
Overseas Competition->Knowledge Point->Calculation Modules->Power->Computing Powers
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 $$7$$. 