Constructing passageways near the dams to allow to salmon to swim around blocked rivers. Thus, option "D" is correct.
The construction of dams in rivers has blocked the path of some salmons returning to spawning grounds.
The best way to overcome this is to make the passage ways near the dams to allow salmons to swim in areas which have blocked due to dams. So that salmons can returned to the spawning ground and can spawn which leads to increase in their population.
Thus, option "D" is correct.
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