Around 1900, fishermen in the Chesapeake Bay area landed more than seventeen million pounds of shad in a single year, butby 1920 , over-fishing and the proliferation of milldams and culvertsthat have blocked shad migrations up their spawning streams had reduced landings to less than four million pounds.
(A) that have blockedshad migrations up their spawning streams had reduced landings to less
(B) that blocked shad from migrating up their spawning streamshad reduced landings to less
(C) that
...
(A) that have blockedshad migrations up their spawning streams had reduced landings to less
(B) that blocked shad from migrating up their spawning streamshad reduced landings to less
(C) that
...
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