How does the ionosphere affect the range of radio communications?

The ionosphere is a region of the upper atmosphere where there are large concentrations of free ions and electrons. While the ions give the ionosphere its name, but it is the free electrons that affect the radio waves and radio communications. In particular the ionosphere is widely known for affecting signals on the short wave radio bands where it "reflects" signals enabling these radio communications signals to be heard over vast distances. Radio stations have long used the properties of the ionosphere to enable them to provide worldwide radio communications coverage. Although today, satellites are widely used, HF radio communications using the ionosphere still plays a major role in providing worldwide radio coverage.

The ionosphere extends over more than one of the meteorological areas, encompassing the mesosphere and the thermosphere, it is an area that is characterised by the existence of positive ions (and more importantly for radio signals free electrons) and it is from the existence of the ions that it gains its name.

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the answer is b, have negligible mass