past continuous

I agree with the OUP answer.

Why can't we use "ate"? Wouldn't the sentence mean "First I arrived, then she ate." if we used "ate"? Or do you mean "ate" is possible but "was eating" more suitable?

I can't answer for Mr. M. but either one is fine in my opinion, and with "ate" it means exactly what you suggest, Diamondrg. The following may be easier to understand that way.

She spoke with me when I arrived.
She gave me the money when I arrived.

CJ

And in Calif's examples, it would definitely sound odd to have the past continuous in the clause before "when I arrived"...

Thank you Jim and Pieanne. I must say that MrM's answer was too concise for me.

Then, Jim, why do you think have they given two equally correct choices in the alternatives. Even more surprising than this is that they have considered "ate" to be wrong. Why? I know that you may not know the exact reason but I just wonder your (and anyone's)opinion.

"When" is a really messy conjunction. It means sometimes "as soon as", sometimes "at the very moment when" and sometimes "during the time in which".
She kissed me when I arrived.
She was kissing him when I arrived.
I arrived when she was kissing him.

As for "She ate when I arrived", I feel some oddness in it. Intransitive "ate" could stand alone when

I would surely not pick 'ate' if you are asked to pick only one of the two options. If there had been a third option-- 'both of the above', then one would be better off picking that. Otherwise, 'ate' seems to require more contextual specificity than 'was eating'. 'Ate' isn't totally wrong, though, as has been pointed out.

This is an aspect of English that is not well documented. In other languages it is known as "Aspect". Eastern Europeans will understand this as the Slavic languages have both perfective and imperfectiove forms of verbs. Other languages too have similar well-formed strutures for expressing aspect.

Aspect basically determines whether you are seeing an action from the inside or the outside.

Then, Jim, why do you think have they given two equally correct choices in the alternatives.

It is probably because the pattern with the past continuous comes to the mind of the native speaker more quickly than any other pattern that it is considered "more correct" than the other pattern. So I'm not sure most people would regard them as "equally correct".

Thank you. It was really illuminating.