Answer Happy

Which of the following is the correct format for a custom campaign parameter added to a URL?

A. www.googlemerchandisestore.com/?utm campaign=fallsale
B. www.googlemerchandisestore.com/?utm_campaign=fallsale
C. www.googlemerchandisestore.com/?utm_campaign=fall_sale
D. www.googlemerchandisestore.com/?utm_campaign = fall sale