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