如题所述
因为/i/是前元音，C和D是冲突的C是说/i/是中元音。
如下是胡壮麟新编语言学教程第二章第五节供参考：
2.5 Vowels
2.5.1 Four criteria of vowel description
the part of the tongue that is raised: front, central, back;
the height the tongue raising: high, mid (often divided into mid-highand mid-low, low）
the mouth openness: close, close-mid, open-mid, open;
the length or tenseness of the vowel: long vs. short, or tense vs. lax;
lip-rounding: rounded vs. unrounded or spread.
如下是胡壮麟新编语言学教程第二章第五节供参考：
2.5 Vowels
2.5.1 Four criteria of vowel description
the part of the tongue that is raised: front, central, back;
the height the tongue raising: high, mid (often divided into mid-highand mid-low, low）
the mouth openness: close, close-mid, open-mid, open;
the length or tenseness of the vowel: long vs. short, or tense vs. lax;
lip-rounding: rounded vs. unrounded or spread.
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