引起牙周脓肿最常见的病原菌是
A．甲型溶血性链球菌
B．类白喉杆菌
C．无芽胞厌氧菌
D．绿脓杆菌
E．白念珠菌(白假丝酵母菌)
参考答案：C
A．NH3、HCl
B．N2、O2
C．H2、CO
D．CO2、O2
—This digital camera is too expensive. I can't afford it.
—Here's another one, madam. It's _________. [ ]
A. better
B. dearer
C. cheaper
