A classic error

One of the things you learn as an instructor is how to write tests - and for multiple-choice tests, that includes writing the distractors - the wrong answers. Here's a question from a Facebook quiz that illustrates one of the classic errors:

Do you see it? The grammar forces you to the right answer. "A ten thousand words" and "a unlimited words" aren't possible English phrases. The 'a" belongs with the answer, not as part of the question.

(I certainly have quibbles with the quiz, though: since when is "When the going gets tough, the tough get going" a proverb, let alone a "most important" one?)