the question whether of + 句子，这样的用法对吗？

白勇GMAT语法全解分词和不定式的的第15题，答案选D

In an assessment of the problems faced by rural migrant workers, the question of whether they are better off materially than the urban working poor is irrelevant.

有点疑惑，这里 the question of whether ... 如何解释？不应该是 "the question whether ..."吗？

谢谢！