When should a writer use a dash?

A. To set off an introductory word or phrase
B. To indicate an abrupt shift in idea.
C. To separate items in a list.
D. To separate clauses in a compound sentence.

All of the above. A writer can use a dash to set off an introductory word or phrase, indicate an abrupt shift in idea, separate items in a list, and separate clauses in a compound sentence.