Which of the following strategies is best for implementing bulk e-mailing to customers?

Which of the following strategies is best for implementing bulk e-mailing to customers?

A.
Generate a list of e-mail addresses from a directory listing of people who work in the same field as your company.

B.
Buy an e-mail list from a vendor that sells a similar product.

C.
Give customers the option to receive or to stop receiving bulk e-mail.

D.
Send e-mail messages to all customer e-mail addresses collected at the time of purchase.