We can never expect ________ bluer sky unless we create ________ less polluted world. [ ]

A. a; a  

B. a; the  

C. the; a  

D. the; the

牙釉质牙骨质界正确的是

A．牙釉质和牙骨质端一端相接占10%
B．牙釉质覆盖牙骨质少许占30%
C．牙釉质覆盖牙骨质少许占60%
D．牙骨质覆盖牙釉质少许占60%
E．牙釉质和牙骨质分离占30%