单项选择题
A.abc，A_4d，_student，xyz_abc B.auto，12-a，a_b，ab5.x C.A_4d，_student，xyz_abc，if D.abc，a_b，union，scan
A.’2’ B.’ab’ C.’A’ D.’’
A.int和float B.float和double C.int和double D.float和char