The World Expo (世博会) __________ in Shanghai now. [ ]

A. being had

B. is having

C. is holding

D. is being held

患牙多见于上、下颌前磨牙和上颌恒切牙。病变为乳牙有关的感染或创伤引起继生恒牙成釉细胞的损伤，导致继生恒牙釉质形成不全或矿化不全。该疾病是

A．氟牙症
B．釉质形成缺陷症
C．Turner牙
D．先天性梅毒牙
E．釉质浑浊症