It often happens that people with good eyesight fail to see _____ is right in front of them. [ ]

A. which

B. that

C. what

D. as

牙龈因失去食物的按摩而软弱无力，引起牙龈萎缩，其原因是

A．牙冠形态无凸度

B．牙冠形态凸度过小

C．牙冠形态凸度过大

D．牙冠形态凸度正常

E．以上都错