通过下图面积的计算，验证一个恒等式，此等式是
A. ${{a}^{2}}-{{b}^{2}}=\left( a+b \right)\left( a-b \right)$
B. ${{\left( a-b \right)}^{2}}+4ab={{\left( a+b \right)}^{2}}$
C. ${{\left( a-b \right)}^{2}}={{a}^{2}}-2ab+{{b}^{2}}$
D. ${{\left( a+b \right)}^{2}}={{a}^{2}}+2ab+{{b}^{2}}$