下列计算正确的是(   )

A：$\frac{2}{m}$ + $\frac{1}{-m}$ = $\frac{3}{m}$

B：$\frac{2}{y+2}$ - $\frac{1+y}{2+y}$ = $\frac{1+y}{y+2}$

C：$\frac{a}{a-b}$ - $\frac{b}{b-a}$ =1

D：$\frac{a}{{\left(a-b\right)}^2}$ - $\frac{b}{{\left(b-a\right)}^2}$ = $\frac{1}{a-b}$