In Which of the Following Instances Do You Usually Have

In which of the following instances do you usually have to use a combining vowel?

A) When connecting a prefix to a root that begins with a consonant
B) When connecting two word roots
C) When connecting a word root and a suffix that starts with a consonant
D) When connecting two word roots AND when connecting a word root and a suffix that starts with a consonant
E) All of these