在gmat中，单位词都是用less修饰；
having done的确不能做限制性定语；
C，by 1920提示完成时，时态错误
A，block... from doing

having done不能作定语，DE错

specifically, you don't use "fewer than" with NUMBERS + UNITS OF CONTINUOUS MEASURE, such as gallons, years, feet, liters, etc.
this isn't really an exception to the rule, anyway, since those units fail both of the criteria above. for instance, if i have N gallons of something, then (1) fails because i can easily have a decimal number of gallons, and (2) of fails because the gallons are not physically separable quantities -- just one big mass of fluid.

for instance, people, shirts, books, etc. all satisfy these two criteria. you can't have decimal quantities of any of those things, and each unit is separable from the other units.

you only use "fewer than" with what are called countable items.
strictly speaking, this means things that:
1) ONLY exist in whole number quantities (i.e., you can't have, say, 5.5 of them), and
2) are actually separable from each other.

A选项：have blocked shad现在完成时态修饰发生在1920年之前的事情有问题；migrations up...应该用V;
B选项：时态和动词使用均无问题；
C选项：时态过去时没有过去完成时好；同时a lower amount 表达wordy,可以直接less than...
D选项：have的现在分词前面应该有逗号；过去完成时会使表达效果更好；
E选项：have的现在分词前面应该有逗号；migrations，to an amount lower错误同上；

E错：1.完成时态having done不能作修饰
2.amount错，只能是数字的比较，而句子后面出现了pounds，带单位了

migrations up their spawning streams这个是怎么个不对的法？

having been done 和having done不能作修饰语？pending

less than 是充当形容词，没有比较

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

A the present perfect tense of have blocked inappropriately describes an event that caused something to happen before 1920

前面的more than，后面的less than

OG2017官方解释


Diction; Verb form
The point of this sentence is to explain how overfishing and interference with shad
spawning streams affected the size of shad landings. The sentence makes this point by
comparing the sizes of annual landings before and after 1920. The sentence most
efficiently compares the poundage of pre- and post-1920 landings with the comparative
form [from] more than . . . to less than. . . .

官方解释
续

A. The present-perfect tense of have blocked inappropriately describes an event that
caused something to happen before 1920. In addition, migrations up their
spawning streams is incorrect.

续

B. Correct. The comparison of poundage is efficiently explained, and the sequence of
tenses makes sense. Despite a possible superficial appearance of a comparison
between countable things (pounds), less is more appropriate than fewer for the
comparison. The fishermen landed different amounts of fish; they did not land the
numbers of pounds in terms of which those amounts are measured.

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.

续：


E. The present-perfect participial phrase must be set off with commas; the pronoun
their, which is also in (A), nonsensically refers to migrations, and the comparative
expression to an amount lower is unnecessarily wordy

A, have blocked 时态错误
C. a low amount 不可数修饰 4m pounds可数，错误
D.having blocked 错了吗？
E. having blocked 错了吗？an amount lower错误同C

B：reduced landings to less than four million pounds
C：reduced landings to a lower amount
减少到了多少磅，而不是减少到了一个数量