The electron density of a layer of ionosphere at a height \[150\text{ }km\] from the earth's surface is \[9\times {{10}^{9}}\,per\,{{m}^{3}}.\]For the sky transmission from this layer up to a range of \[250\text{ }km,\]The critical frequency of the layer is

• A. \[2\,Hz\]
• B. \[2.7\,Hz\]
• C. \[2.78\,Hz\]
• D. \[2.7\,MHz\]