元杂剧的结构通常由（）构成。
A.一折
B.二折
C.三折
D.四折
参考答案：D
He hurried to the booking office only________that all the tickets had been sold out.[ ]
A. to tell
B. to be told
C. telling
D. told
下列各组中的性质比较，正确的是( )
A．酸性HClO4>HBrO4>HIO4
B．碱性NaOH>Mg(OH)2>Ca(OH)2
C．稳定性：HI>H2S> HCl
D．氧化性：Na+>Mg2+>Al3+
