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.