龈沟底在釉牙骨质界的根方，同时龈沟深度超过多少称为牙周袋
A．2mm
B．2.5mm
C．3mm
D．4mm
E．6mm
参考答案：C
下列物质间的关系中，不能直接相互转化的是（ ）
A．CO2-H2CO3
B．CO2-CaCO3
C．CO2-Ca（OH）2
D．CO-CO2
—Kevin, is your twin brother the same as you?
—No. I'm ______ than him. [ ]
A. a little thin
B. very thinner
C. much thiner
D. a little thinner
