A.N5-CH-NH-FH4 B.N5-CH3-FH4 C.N10-CHO-FH4 D.N5，N10-CH2-FH4 E.N5，N10=CH-FH4
A.5’-ATP B.3’-ATP C.3’-dATP D.5’-UTP E.5’-dUTP
A.XMP B.IMP C.GMP D.AMP E.CMP
