下列计算正确的是（ ）

$A:\sqrt{8}-\sqrt{2}=\sqrt{2}$

$B:\left(2-\sqrt{5}\right)\left(2+\sqrt{5}\right)=1$

$C:\dfrac{6-\sqrt{2}}{\sqrt{2}}=3\sqrt{2}$

$D:\dfrac{\sqrt{27}-\sqrt{12}}{3}=\sqrt{9}-\sqrt{4}=1$