Dear Professor,
I have got a question about a grammar problem. My friend had a discussion with a teacher about this sentence: "Julia .............. just................... (cover) herself in sun tan lotion when the sun ................... (go) behind a large black cloud."
She wrote: "Julia was just covering herself in sun tan lotion when the sun went behind a large cloud."
The teacher, however, says it should be: "Julia had just covered herself in sun tan lotion when the sun went behind a large black cloud."