"E" is the correct choice - study happened in the past, so perfect tense must be used.
"A" and "B" are out due to verb+ing rule
C - 'having failed' does not make sense
D - "such variables like..." incorrect usage of such X like Y. It must be 'such X as Y.
Hope this helps.
"A" and "B" are out due to verb+ing rule
C - 'having failed' does not make sense
D - "such variables like..." incorrect usage of such X like Y. It must be 'such X as Y.
Hope this helps.
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