There ____________ always three lights in every country: red, yellow ____________ green.[ ]

A. is; and

B. are; and

C. are; or

儿童最常见的脊髓肿瘤是（）。

A.髓母细胞瘤

B.少突胶质细胞瘤

C.星形细胞瘤

D.生殖细胞瘤

E.畸胎瘤