Northern Trail Outfitters (NTO) has created a new onboarding series for customers who have purchased its fitness tracker. Customers were automatically opted in and received three emails from NTO within one day. The onboarding series includes personalized recommendations but lacks an easy way to opt out or update preferences.

In this scenario, what best practice was achieved?

A. Sending multiple emails in one day

B. Personalized recommendations

C. Opting customers in automatically
D. Preventing customers from opting out