a. Jane had been studying for 5 hours when her parents came home.
b. Jane was studying for 5 hours when her parents came home.
I think 'a' is ok, but what about 'b'? As a non-native, I wonder 'b' is correct, especially in grammar. Can I use past continous with 'for'?
A native teacher replied to me just a while ago that 'b' is not correct. "In spoken informal English, yes, you can use the Past Continuous and there will no misunderstanding. However, in a test, if you want to be perfectly grammatically correct, you should use the Past Perfect Tense. A well-spoken native speaker would also use the Past Perfect Continuous if they mention the length of time."
What's your opinion about this? Is 'b' natural or awkward?