Which sentence correctly uses the past perfect continuous tense to describe an action continuing over a period up to a specific time in the past?

A. She had been working as a clerk for 10 years before she retired.

B. She has been working as a clerk for 10 years before she retired.

C. She was working as a clerk for 10 years before she retired.

D. She had worked as a clerk for 10 years before she retired.