What is the key difference between past perfect and past perfect continuous tenses?

A. Past perfect tense describes actions completed before a certain point in the past, while past perfect continuous tense emphasizes the duration of an action that started before a certain point in the past and continued up to that point.

B. Past perfect tense describes ongoing actions in the past, while past perfect continuous tense describes completed actions in the past.

C. Past perfect tense describes actions happening in the present, while past perfect continuous tense describes actions completed before a certain point in the past.

D. Past perfect tense describes actions starting in the future, while past perfect continuous tense describes actions happening before now.