Which statement is correct for the reaction below?

4x+z yields 2r+2s
a: the rate of formation of r is one-half the rate of the disappearance of z.
b: the rate of disappearance of z is one-fourth of the rate of disappearance of x.
c: the rate of formation of r is double the rate of formation of s.
d: the rate of formation of s is double the rate of disappearance of x.