C: To ask a question
If it ends in a period, it is most likely to make a statement, express a feeling, or to give an order. Examples:
Fish swim in water.
I'm very happy.
Go clean your room now.
If it ends in a question mark it is to ask a question. Examples:
Are you happy?
if it ends in an exclamation point it is to either give an order or express a feeling (usually of excitement or surprise). Examples:
Go clean now!
I'm very happy!
Ah! (surprise)