赞题库-背景图
单项选择题

已知z=sin(3x+y), 则dz=( )
A、3cos(3x+y)dx+cos(3x+y)dy
B、cos(3x+y)dx+cos(3x+y)dy
C、3cos(3x+y)+cos(3x+y)
D、3cos(3x+y)dx+sin(3x+y)dy