To train dogs to fetch, trainers often start with a_____.

　　细节理解题。

　　定位到文中第四段的第一、二句，“Teaching your dog to fetch promotes bonding. To do this, first throw a ball, and give the command "fetch".”，教你的狗去捡东西。这样做，先扔出一个球，然后给出“取来”的指令，故选B。