1. Among the following four equations, the incorrect one is ( ).
(A) $1-a-b+a b=(1-a)(1-b)$
(B) $1-a+b+a b=(1-a)(1+b)$
(C) $1+a+b+a b=(1+a)(1+b)$
(D) $1+a-b-a b=(1+a)(1-b)$
-、1.(B).
By swapping the shadows of $a$ and $b$, it can be found that (B) and (D) cannot be converted into each other, so one of them must be wrong. Also, by expanding $(1-a)(1+b)$, there should be two minus signs $-a-ab$, indicating that (B) does not hold.