请教OG18 262

有很多可能的L

各位大大好~
想请教一下下面这题：
Around 1900, fishermen in the Chesapeake Bay area landed more than seventeen million pounds of shad in a single year, but by 1920, over-fishing 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.
(A) that have blocked shad migrations up their spawning streams had reduced landings to less
(B) that blocked shad from migrating up their spawning streams had reduced landings to less
(C) that blocked shad from migrating up their spawning streams reduced landings to a lower amount
(D) having blocked shad from migrating up their spawning streams reduced landings to less
(E) having blocked shad migrations up their spawning streams had reduced landings to an amount lower

先说下我的问题：为什么这里用的是their，虽然这个不是考点，但也麻烦大家给分解答哈


附上我总结的关于这道题的知识点~

262	Meaning is everything！！！             i)Less than 20 dollars--less than the amount of money强调money                            ii)Fewer than 20 dollar bills--actual pieces of paper强调那纸	i)Fewer pounds of shad--different numbers of pounds   强调不同的数额的 pound      ii)Less pounds of shad--       different amount of fish      强调shad, the fish       Less is more appropriate	Shad: a kind of fish   红鲱鱼
	小心所有格's和代词同时出现的情况      代词指代的antecedent必须是N 不能是ADJ.	EG:have blocked Shad migrations up their spawning streams 这里的shed做的是形容词，本身这个their应该指代的是shad，这里因为shad作形容词，their错误指代migrations	EG：Shakespeare's
		正确：blocked shad from migrating up their spawning streams
	But前后可以用两种时态，前提是必须有时间标志词	EG:Around 1900 过去式,      but by 1920 过去完成时	EG：过去式 in the early 1980s, but less than ten years later, 过去完成时
	现在完成时/过去完成时：                 i）Continued action:过去的动作持续到现在这个点/过去的过去的动作持续到过去那个点                             ii）Continued effect:过去的动作或行为的影响持续到现在/过去的过去的动作或行为的影响持续到过去	此题为什么用过去完成时而不是过去时：                             1) By 1920 时间标志词                 2） overfishing和proliferation of milldams and culverts的影响持续到1920（过去的那个时点）	i)"Less than ten years later" 以“ten year later"为1980（原题给的）的后十年来说，less than that意味着在过去（1980+10）往回推，因为不到十年                    ii)Continued effect原文句意很有意思 在曼哈顿SC纸质总结Verbs第八章里

有很多可能的L

遗留未解知识点：为什么后面用less这里用their，their肯定指代的是shed，按shed的不可数来说，因为后面用的less，这里是不是用i更合理呢？