Which Statement Is TRUE Regarding Use of a Combining Vowel

Which statement is TRUE regarding use of a combining vowel?

A) When a word root is connected to a suffix,the combining vowel is always used.
B) When two word roots are connected,the combining vowel usually is not used.
C) A combining vowel is not used when a prefix and a word root are connected.
D) A combining vowel is most frequently an i,and only rarely is an o or an e