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$$\overline{**45}$$, $$\overline{19*8}$$, $$\overline{23*1}$$, and $$\overline{3*49}$$ are four $4-$digit numbers with some unknown digits. Which number is possible to be a perfect square? 
Overseas Competition->Knowledge Point->Number Theory Modules->Perfect Square Numbers
According to the ones digit of perfect squares, we can eliminate choice $B$.  A perfect square ending with $5$ must be a multiple of $25$, so we can eliminate choice $A$.  $${{48}^{2}}=2304$$ and $${{49}^{2}}=2401$$ so we can eliminate choice $C$.  ${57}^{2}=3249$. We choose option $D$. 