The inertia constant of the two machines which are not swinging together are M1 and M2. The equivalent inertia constant of the system is:

A

$$M1 - M2$$

B

$$M1 + M2$$

C

$$\frac{M1 M2}{(M1 + M2)}$$

D

$$\frac{M1 M2}{(M1 - M2)}$$