If z=x+iyz = x+iyz=x+iy is a purely imaginary number, which of the following must be true?
x=0,y=0x=0, y=0x=0,y=0
x=0,y≠0x=0, y \neq 0x=0,y=0
x≠0,y=0x \neq 0, y=0x=0,y=0
x=yx=yx=y