A．<TABLE cellSpacing=0 cellPadding=0 width="90%" border=0>
<TBODY>
<TR>
<TD vAlign=top>两铰拱 </TD></TR></TBODY></TABLE>
B．<TABLE cellSpacing=0 cellPadding=0 width="90%" border=0>
<TBODY>
<TR>
<TD vAlign=top>无铰拱 </TD></TR></TBODY></TABLE>
C．<TABLE cellSpacing=0 cellPadding=0 width="90%" border=0>
<TBODY>
<TR>
<TD vAlign=top>三铰拱</TD></TR></TBODY></TABLE>