Let G be the geometric mean of two positive numbers a and b, and M be the arithmetic mean of $\;\large\frac{1}{a} \;$ and $\;\large\frac{1}{b}\;$ . If $\;\large\frac{1}{M} : G \;$ is 4 : 5 , then a : b can be :

Question

$(a)\;1 : 4\qquad(b)\;1 : 2\qquad(c)\;2 : 3\qquad(d)\;3 : 4$