Bobby was a dog that went every moming to a baker's shop to fetch twelve bread roll for its owner. However. when Bobby returned back one day, it had only eleven rolls. Bobby's owner wants to know what had happened, but he followed Bobby to the baker's shop the next day Instead taking the road home as usually. Bobby took another road and went up to a farm. There was a poorer little dog with a broken leg. Bobby was offered the roll to the little dog. The owner was very proud of Bobby and rewarded itself a nice big bone. Later, the owner's family never grew tired of tell everybody how kind and clever Bobby was

小学生应重点保护的牙是

A．乳切牙

B．乳磨牙

C．恒切牙

D．第一恒磨牙

E．第二恒磨牙