面有横嵴的牙是（）
A.上颌第一磨牙
B.下颌第一磨牙
C.上颌第一前磨牙
D.下颌第一前磨牙
E.下颌第三磨牙
参考答案：D
与3.2gSO2所含的氧原子数相等的NO2的质量为（ ）
A．3.2g
B．4.6g
C．6.4g
D．2.3g
Nowadays it is not uncommon for a big, p schoolboy to elbow an elderly woman aside in the dash for the last remaining seat on the bus or tube, () stand up and offer his seat to her.
A. much less
B. not to mention
C. not to say
D. rather than
