6. If $a$ and $b$ are the roots of the equation
$$
(x+c)(x+d)=1
$$

then, $(a+c)(b+c)$ equals ( ).
(A) 1
(B) -1
(C) 0
(D) $c^{2}$
6. B.'

From the given, we have $\left\{\begin{array}{l}a+b=-(c+d), \\ a b=c d-1 .\end{array}\right.$
$$
\begin{array}{l}
\text { Then }(a+c)(b+c)=a b+(a+b) c+c^{2} \\
=a d-1-(c+d) c+c^{2}=-1 .
\end{array}
$$