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题库 / OG2017V-SC-226
Around 1900, fishermen in the Chesapeake Bay area landed more than seventeen million pounds of shad in a single year, but by 1920, overfishing and the proliferation of milldams and culverts that have blocked shad migrations up their spawning streams had reduced landings to less than four million pounds.
that have blocked shad migrations up their spawning streams had reduced landings to less
that blocked shad from migrating up their spawning streams had reduced landings to less
that blocked shad from migrating up their spawning streams reduced landings to a lower amount
having blocked shad from migrating up their spawning streams reduced landings to less
having blocked shad migrations up their spawning streams had reduced landings to an amount lower
by 1920, 有一个明确的过去的时间，动词用had reduced，排除 cd
ae 里their指代的是migrations 不合理
应该是这个样子的，但也不是很明白，求解析
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