The correct choice is (a) True
Easiest explanation: The G.C.A includes the cultivated as well as an uncultivated area like ponds, residential areas, roads, etc. An irrigation canal lies in the area between the two drainages and can economically irrigate it. Thus, the drainages fix the boundary of the gross command of a canal system and it becomes uneconomical to use the system across the two drainages i.e boundaries.