Tính \(d\left( {\sin x - x\cos x} \right)\).

A. \(d\left( {\sin x - x\cos x} \right) = x\sin xdx\)

B. \(d\left( {\sin x - x\cos x} \right) = x\cos xdx\)   

C. \(d\left( {\sin x - x\cos x} \right) = \cos xdx\)

D. \(d\left( {\sin x - x\cos x} \right) = \sin xdx\)