Which of the following is not a tip for professional e-mail etiquette?

A) Be concise and to the point.
B) Use spell-checker and proofread before sending.
C) Use abbreviations such as "u," "r," and "BRB" to keep the e-mail message shorter.
D) Include a meaningful subject line.

Using abbreviations such as "u," "r," and "BRB" is considered informal and not appropriate for professional email communication. Professional emails should maintain a level of formality and clarity, avoiding slang and abbreviations that might not be understood by all recipients or that might convey an unprofessional tone.