Marketing by emailing is on the increase. Compared to other media, email messages are extremely cheap to send. With TV, you are spending on ad agencies and cable channels. With print ads, you are helping to keep newspapers and magazines alive. Direct mail costs more than $600 per thousand pieces. With email, there are almost no costs at all. Its low cost makes email marketing become the most cost-effective (划算的) advertising method available today. With TV, you do not know who is watching your ads. Even with direct mail, you cannot be sure that your mail has been delivered, or that anyone reads it when it gets there. With email, you know within 24 hours exactly which messages have been opened, by whom, what links the opener clicked on, and what part of your message was working.If you market by emailing, you can learn within a day ___________.