Explanation: VHF signals typically travel line-of-sight. Atmospheric ducting and tropospheric propagation describe conditions where abnormal temperature or humidity gradients in the lower atmosphere (troposphere) create layers that refract or bend radio waves. This can cause VHF signals to be trapped within atmospheric "ducts" or refracted back towards the Earth beyond the normal radio horizon, allowing them to travel significantly farther than usual. Changing power (B) increases signal strength but doesn't alter the fundamental propagation mode to achieve "much longer" distances. Skywave reflections from the D layer (C) and ionospheric activity in layers F1/F2 (D) are mechanisms primarily associated with HF (High Frequency) propagation, not VHF. VHF signals generally pass through the ionosphere into space, rather than being reflected by it under normal conditions.