Which Statement Best Describes an Appropriate Opening for an Unsolicited

Which statement best describes an appropriate opening for an unsolicited cover letter?

A) If you are unsure whether a position exists, use a more persuasive opening.
B) Avoid mentioning the company by name so that you can send your cover letter to numerous companies.
C) Make cover letters for unsolicited jobs longer than those for solicited jobs because you will have to provide more information about yourself.
D) Be vague about the type of position you're seeking so that you might be considered for a variety of jobs.