If there's no reference to A, yes, that is plagiarism. If Y has not read the papers by B and C, it's also reference padding.
It is possible that Y did read those papers by B and C and happened upon the same sentence structure as A, which is not plagiarism. That's actually pretty easy when citing one paper. "According to Smith (2001) blah, blah..." happens all the time. With two authors and 25 words, it is much less likely that Y independently wrote the same thing.
You don't tell us the context. I suppose it's possible there is a special case that would mitigate this, but I can't think of one.