Identify the sentence with correct use of the past perfect tense:
A. By the time I arrived, they had already left.
B. By the time I arrived, they already leave.
C. By the time I arrive, they had already left.
D. By the time I arrive, they already left.
The correct form is "By the time I arrived, they had already left," using the past perfect tense for an action completed before another past action.