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Grammar/Sentence Correction - Use of Past Perfect

Let's take a look at an official GMAT Prep question today which confuses people about the use of past perfect. In our module, we have discussed all situations in which we use the various tenses. We use past perfect to show past in the past. When we use words such as "before" or "after" or we use years/dates etc, we do not need to use past perfect.

 

Now look at this official question:

 

Question: 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


The correct answer here is (B).


Let's look at all the decision points one by one:

 

that blocked/that have blocked/having blocked

 

'that have blocked' because we are talking about something that happened a century ago. So present perfect (recency) will not work. 

'having blocked' is perfect participle modifying milldams and culverts. It needs to show a completed action before another such as "Having eaten a whole pizza, I skipped dinner."

The action of blocking was not complete before the action of reduction happened. The blocking was causing reduction. Hence 'having blocked' is incorrect.

'that blocked...' is a relative clause modifying milldams and culverts correctly. 

 

had reduced/reduced

 

Look at the structure of this entire clause: By 1920, A and B had reduced landings to less than four million pounds.

So we have a point in time in the past which is 1920. Before 1920, A and B had caused reduction such that by 1920, landings had become less than 4 million pounds. Since it is apparent that the reduction took place in the years from 1900 to 1920, we will use past perfect for reduction - an action before a certain point of time in the past. 

Hence, past perfect is correct here. A construction such as “by year X, …” often uses past perfect tense. 

Note what would have happened if the sentence were a little different. 

Around 1900, fishermen in the Chesapeake Bay area landed more than seventeen million pounds of shad in a single year, but in 1920, they landed less than 4 million pounds of shad.

We don’t need to use past perfect in this sentence. We are given the years so it is clear what happened before and what happened later. 

 

less than/lower amount than/an amount lower than

 

A question that arises here sometimes is why “less than 4 million pounds” is acceptable when pounds are countable. When we have a threshold of value, and the value need not be an integer, use of “less” is acceptable such as 

She earns less than 2 million dollars per year. 

With measurements which needn’t be integers, we use “less than”.

 

We use “lower than” when we compare figures such as

Today, my cholesterol is lower than what it was two years ago. 

 

Hence (B) satisfies all the parameters and is the answer. 

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