In a particular dialect of English, the sounds [s] and [ʃ] n…

In a particular dialect of English, the sounds [s] and [ʃ] never occur in the same phonetic environment: [s] appears before front vowels (e.g., [i, e]) and [ʃ] appears before back vowels (e.g., [u, o]). Based on this distribution, what can you conclude about the phonological relationship between [s] and [ʃ] in this dialect?