The correct answer is B: The high gain antennas commonly used can result in high exposure levels. A hazard created by operating at microwave frequencies is that the high gain antennas commonly used can result in high exposure levels. Microwave antennas often have very high gain, concentrating RF energy into narrow beams with high power density. Microwave frequencies (above 1 GHz) often use high-gain antennas like parabolic dishes, which can have 20-40 dBi gain or more. These antennas concentrate power into very narrow beams. If someone is in the main beam, they can be exposed to very high power density even with moderate transmitter power. For example, 10 watts into a 30 dBi dish creates a very intense beam. This is why microwave operations require careful evaluation of exposure, especially in the main beam direction. The high gain creates localized high exposure levels that must be managed.