Differentiate between a modal and auxiliary verb.

A. Modal verbs are always followed by a main verb, while auxiliary verbs can stand alone.

B. Modal verbs are used for forming questions, while auxiliary verbs are used for making statements.

C. Modal verbs express necessity, possibility, or permission, while auxiliary verbs are used to form tenses and voice.

D. Modal verbs express past actions, while auxiliary verbs express future actions.