She <span>had been working</span> on the project for 2 days before she finished it. So B is the only right one as it's a continuous action before an action with an exact defining past perfect continuous. Also there's a key word "for" that can help us to find out the tense.
Romeo and Juliet's tragic deaths
The novel was so compelling that I couldn't put it down.
The recent food-poisoning scare has reignited debate over farming methods.
