Between A&C the only difference is the tense the two options use towards the end of the sentence. Both options begin with present perfect "it has been trying" but then differ.
Ms. Omari's resignation will leave the organization with another major executive opening at a time when
it has been trying to replace its longtime Chief Operating Officer, who, last year, left to start her own bootstrapped company.
A. it has been trying to replace its longtime Chief Operating Officer, who, last year,
leftC. it has been trying to replace its longtime Chief Operating Officer, who last year,
had leftChoice A uses simple past, where as choice C uses past perfect. Past perfect is used to express that an action occured before another action which also occured in the past. Therefore we would use past perfect for the action which happend first and simple past for the action, which also happend in the past, but happend later.
BUT in this sentence we have present perfect (something which began in the past, but continues into the present or at least the effect of the action continues into the present). So
do we use simple past or past perfect when something occurs before an action in present perfect? The rule for the usage of
past perfect is that it is used to express that an action happend before another action WHICH IS ALREADY COMPLETE. Therefore,
since the action in present perfect IS NOT YET COMPLETE we need to use simple past instead of past perfect.
Therefore
option A is correct, as it uses simple past and not past perfect.
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