A. messages should include no sensitive information.

b. you don't need a subject line.

c. the main idea is revealed immediately.

d. an indirect opener creates a smooth approach.

Most emails and memos should use frontloading, which means that C. the main idea is revealed immediately.

Frontloading refers to the practice where the most important details and information are stated at the very beginning, at an early location in a document. Thus, these emails should be so straightforward by mentioning what they are about in the first couple of lines within the email.