The stratosphere
is the lowest level of the atmosphere
is responsible for Earth's weather
transmits radio signals around the Earth
is used by commercial airplanes

is used by commercial airplanes

Explanation:

The stratosphere is used by most commercial airplanes. This is because this layer of earth is stratified and more stable.

The lowest level of the atmosphere is the troposphere. This layer is where all the bulk of the atmospheric gases are domiciled. Weather conditions occur here. This layer is not safe for flight due to its turbulent nature. The ionosphere is responsible for the transmission of radio signals around the earth. The earth's weather occurs in the region of the troposphere.  

Schist. hope it

That would be a working theory.