Concept:Intervisibility depends on whether any higher ground lies along the straight line joining a viewpoint and the town.
Explanation:From the feature marked H, imagine drawing a straight line of sight to each town.
If elevated land or a ridge rises above that line, the town is hidden.
For Duba, the line of sight from H is blocked by an intervening hill or high land.
For Sasi, Doba, and Oki, no such obstruction exists on their lines of sight.
Therefore, only Duba may not be visible from H.
Answer:Duba
