Which of the Following Is a Difference Between Letters and Memos

Which of the following is a difference between letters and memos?

A) Letters are more formal than memos.
B) Letters are used for analyzing problems, whereas memos are only for formal communication.
C) Letters cannot be sent electronically, whereas memos can be sent electronically.
D) Letters provide a written document for review, whereas memos do not provide a written document for review.