I wasn't sure if he was really interested or if he ________ polite.[ ]

A. was just being  

B. will just be

C. had just been  

D. would just be

牙釉质的无机化合物主要是

A．磷酸钠

B．磷酸钙

C．氟化钙

D．磷酸钾

E．磷酸镁