She had only just put the phone down when the boss rang.

Knowledge: Inverson

Hardly/ scarely/ barely + Had + S + Vp2 when / befefore + S + V(qk) + St

She had just put down the phone when the boss called

A. She put down the phone and the boss called (wrong)

B. As soon as she put down the phone, the boss called (correct in meaning and structure)

C. The boss called but she put the phone down (wrong)

D. She put the phone down, so she let the phone ring when the boss called (wrong)