select the most appropriate word to fill in the blank. Susan finally arrived at the party at midnight, but by then, most of the guests ________. (a) having left (b) have left (c) had left (d) left

Options

A. having left
B. have left
C. had left
D. left

Explanation

had left
RULE: If two actions are carried out in the past and one action has been completed before another, then the first part is the PAST PERFECT TENSE and the second part is in t he SIMPLE PAST TENSE.
In the given sentence, the second part of the sentence is the first action, so “had+ III form” should be used.