氟牙症的主要病因是（）
A.婴儿期高热性疾病
B.人体氟摄入量过高
C.发育期缺微量元素
D.人体氟摄入量过低
E.母亲妊娠期的疾病
参考答案：B
He _____ in the classroom just now. he _____ be there now. [ ]
A. heard to sing, may
B. was heard sing, must
C. heard sing, must
D. was heard to sing, may
肺性骨关节病
