Why is it considered bad manners to leave the subject field blank? a. if there is no subject, recipients cannot use the reply all feature to respond to the e-mail. b. if there is no subject, the e-mail cannot be sent to more than one recipient. c. if there is no subject, the recipient has to open the e-mail to see what it is about. d. if there is no subject, the e-mail sends more slowly than an e-mail with a subject.