${\text{U}}_{\text{rms}}$ of an ideal gas$\text{'}\text{A}\text{'}$ at $\text{'}\text{T}\text{'} \text{K}$  is equal to ${\text{U}}_{\text{mp}}$ of another ideal gas $\text{'}\text{B}\text{'}$at $\left(\frac{\text{T}}{2}\right) \text{K}$. The ratio of their molecular masses, ${\text{M}}_{\text{A}} : {\text{M}}_{\text{B}}$ is :