Cho \(f\left( x \right),\,g\left( x \right)\) là hai hàm số liên tục trên \(\mathbb{R}\). Chọn mệnh đề sai trong các mệnh đề sau.

A. \(\int\limits_{a}^{b}{f\left( x \right)\text{d}x}=\int\limits_{a}^{b}{f\left( y \right)\text{d}y}\).

B. \(\int\limits_{a}^{b}{\left( f\left( x \right)-g\left( x \right) \right)\text{d}x}=\int\limits_{a}^{b}{f\left( x \right)\text{d}x}-\int\limits_{a}^{b}{g\left( x \right)\text{d}x}\).

C. \(\int\limits_{a}^{a}{f\left( x \right)\text{d}x}=0\).

D. \(\int\limits_{a}^{b}{\left( f\left( x \right).g\left( x \right) \right)\text{d}x}=\int\limits_{a}^{b}{f\left( x \right)\text{d}x}.\int\limits_{a}^{b}{g\left( x \right)\text{d}x}\).