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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
续：
官方解释：
C. The comparative expression to a lower amount is unnecessarily wordy. The pastperfect form had reduced would make the temporal relationships somewhat clearer
than does the past tense reduced.
D. The present-perfect participial phrase, having blocked . . . streams, should be set off
in commas; as it stands, it does not make sense. The past-perfect form had reduced
would make the temporal relationships somewhat clearer than does the past tense
reduced.
不明白为什么要用逗号隔开
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