23. Let $f ( x ) = \int \operatorname { ex } ( x - 1 ) ( x - 2 ) d x$. Then $f$ decreases in the interval :\\ (A) $( \infty , - 2 )$\\ (B) $( - 2 , - 1 )$\\ (C) $( 1,2 )$\\ (D) $( 2 , + \infty )$\\