Answer :
The situation in which combining vowel never used is between a prefix and a word root.
What is a combining vowel?
Combining vowels is a letter that is used to ease the pronunciation of medical terminologies.
The letter is usually o. Examples are thermo, cysto, etc. It is used between a suffix and a word root.
Thus, the correct option is C. Between a prefix and a word root.
Learn more about combining vowels
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