GMAT Set 30 - 5

A is the most concise and best choice.Others are wordy and awkward.

E uses extra pronoun word "they". Without this pronoun also, the meaning is clear in A.

A answer choice may be eliminated for pronoun ambiguity if another answer choice avoids the ambiguity and is free of errors.
In B and E, they has two eligible antecedents (economists and users).
Since A avoids this issue and is free of errors, eliminate B and E.
In C, are replacing lacks a subject.
Eliminate C.
In D, it lacks a viable referent.
Eliminate D.
The correct answer is A.

There are 2 prepositions that are confusing – when and “in which” and the confusion is when both of them are used when a reference to time has been made.

“When” is used when there is a specific time or a date that is being discussed.
Ex : 15th August 1947 is the day when India got its Independence.
“In which” is used when we are talking about a time period.
Ex : 1947 was the year in which the freedom struggle came to an end.
Here there is a cycle being referred to. And a cycle is usually a time period. That’s why usage of “which” makes more sense!