What is the last digit of the expression below?  $$\underbrace{2\times2\times2\times\cdots\times2}_{100  ; 2s} \times \underbrace{7\times7\times7\times\cdots\times7}_{100  ; 7s} \times \underbrace{9\times9\times9\times\cdots\times9}_{100  ; 9s}$$ 
Overseas Competition->Knowledge Point->Calculation Modules->Whole Numbers->Calculation of Multi-digit Numbers