We work together to achieve our common purpose：_________world that is safer, cleaner and healthier than         one we found．

A．the；the

B．a；不填

C．a；the

D．the；不填

下述哪一类肿瘤不属于原发性恶性骨肿瘤（）

A.脊索瘤

B.骨纤维肉瘤

C.尤文瘤

D.骨髓瘤

E.成软骨细胞瘤