If the atomic weight of the most stable isotope of the element M is 40 times that of the lightest element, then formula of the compound of its phosphate is

A

$$\text{M}_3(\text{PO}_4)_2$$

B

$$\text{M}_2(\text{PO}_4)_3$$

C

$$\text{MPO}_4$$

D

$$\text{M}_3\text{PO}_4$$