4.[单选题]-|||-设y1,y2是方程 ^n+p(x)y'+q(x)y=f(x) 的解,则 _(1)-(y)_(2) 是下列方程中 () 方-|||-程的解.-|||-(A) '+p(x)y+q(x)=0 (B) ^n+p(x)y'+q(x)=0-|||-(C) ^n+p(x)y'+q(x)y=0 (D) ^n+p(x)y'+q(x)y=f(x)-|||-A (A )-|||-B) ((B)-|||-C ()-|||-D (D )