So wouldn't a more correct answer be "It may print any combination except "a," or "b," or "a, b, " or "b, a""?and so the thread that removes "a" and "b", may remove them in any order. Thus, the iterator thread may or may not see "a" and/or "b" through its Iterator.
I'm not sure this question really tests what you're trying to test, but I'm probably missing something here.