Two non-reactive monoatomic ideal gases have their atomic masses in the ratio `(2 : 3)`. The ratio of their partial pressure, when enclosed in a vessel kept at a constant temperature, is `(4 : 3)`. The ratio of their densities is :
A. `1 : 4`
B. `1 : 2`
C. `6 : 9`
D. `8 : 9`