把多项式 ${\mathit{x}}^{\mathit{2}}\mathit{+}\mathit{ax}\mathit{+}\mathit{b}$分解因式，得 $\mathit{(}\mathit{x}\mathit{+}\mathit{1}\mathit{)}\mathit{(}\mathit{x}\mathit{-}\mathit{3}\mathit{)}$，则 $\mathit{a}$， $\mathit{b}$的值分别是 $\mathit{(}$　　 $\mathit{)}$

A． $\mathit{a}\mathit{=}\mathit{-}\mathit{2}$， $\mathit{b}\mathit{=}\mathit{-}\mathit{3}$B． $\mathit{a}\mathit{=}\mathit{2}$， $\mathit{b}\mathit{=}\mathit{3}$C． $\mathit{a}\mathit{=}\mathit{-}\mathit{2}$， $\mathit{b}\mathit{=}\mathit{3}$D． $\mathit{a}\mathit{=}\mathit{2}$， $\mathit{b}\mathit{=}\mathit{-}\mathit{3}$