In A, it is unclear whether their refers to changes or to conditions.
Eliminate A.
C: The Hawthorne Works WAS the scene...for investigating...what the effects...ARE.
Here, the sequence of events is illogical:
It is not possible that the Hawthorne Works WAS the scene (in the past) for investigating what the effects ARE (in the present).
Eliminate C.
D: changes in working conditions' effects
Here, there are changes in the EFFECTS.
The intended meaning of the original sentence is that there were
...
Eliminate A.
C: The Hawthorne Works WAS the scene...for investigating...what the effects...ARE.
Here, the sequence of events is illogical:
It is not possible that the Hawthorne Works WAS the scene (in the past) for investigating what the effects ARE (in the present).
Eliminate C.
D: changes in working conditions' effects
Here, there are changes in the EFFECTS.
The intended meaning of the original sentence is that there were
...
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