What is the best indicator that a sentence uses the past perfect tense of the verb?

A. The verb is consist of two words: an auxiliary verb and a main verb.
B. There are two verbs which both show past events.
C. One of the two past actions happened earlier than the other.
D. Between the two verbs in the sentence which are both in the past, the one with the auxiliary verb had indicates an action that happened earlier than the other action which is in the simple past.