Can I use past continuous with 'for'?

Question

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'?

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Addition

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?

Answer 1

Yes and no.

Past continuous with "for" is okay. However, "b" is not correct.

"B" is not correct because of "when". Did Jane's parents need 5 hours to arrive at home? No, so Jane's studying happened in the 5 hours before they arrived. Since the studying was before a past event, the sentence needs to be "a", not "b".

Use past continuous with a time expression.

Correct:

"I was tossing and turning all night."

"Jane was studying for 5 hours yesterday."

Answer 2

Both are correct and understandable and have the same meaning that

Jane studied for 5 hours before her parents came home.