On which of the following amateur bands is long-path propagation most frequent?

Объяснение

Long-path signals must stay below the maximum usable frequency over the whole long way round while remaining high enough in frequency to escape crippling cumulative absorption, and 40 and 20 metres meet both conditions most often. The lowest bands take too much absorption over such distances, while 10 metres and above open the long way round only when solar activity keeps the MUF high along the entire route.