What Were You Doing When…?

From Twitter:

Read:
_____ drinking tea only ten minutes before the phone rang.

Choose:
have been
was
had been

Answers & Explanations
The answers are ‘had been’ (formal, correct and written) and ‘was’ (casual and spoken). ‘Have been” doesn’t work because it is in the present; we need a past answer dictated by ‘rang.’

Sarah Gaultieri