What does the term revolving door refer to?

Read this cover letter: dear sir or madam: i am responding to your ad seeking data managers. i feel i am highly qualified for the position and have a great deal of relevant experience. i have managed the bookkeeping data for a local volunteer organization for several years. they couldn't get rid of me; i loved the job so much! i can provide several references attesting to my punctuality, bookkeeping abilities, and reliability. i look forward to your response. best wishes, john smith what is one of the weaknesses of this cover letter? a. it overstates the candidate's qualifications. b. it makes an attempt at humor. c. it provides irrelevant information. d. it makes the candidate appear selfish.