单项选择题

A.紧挨龈缘和黏膜
B.离开黏膜约0.1mm,边缘距龈缘Imm
C.离开黏膜约0.2mm,边缘距龈缘2mm
D.离开黏膜约0.3mm,边缘距龈缘3mm
E.离开黏膜约0.5mm,边缘距龈缘5mm