Which of the following is an example of constructive criticism for a friend who speaks too softly?

A. You are so annoying! You need to use your adult voice in meetings.
B. You should quit mumbling. It is so frustrating to try and understand you.
C. Your voice is so nice. I would love for you to speak up, so I can hear you better.
D. You talk like you are whispering. Speak up so people don't struggle to hear you.