document.write ("<b>上一篇</b>：<a href='/DocHtml/699/2014/1/11/3761655534638.html'>Unit 9   When was it invented？</a><br><b>下一篇</b>：<a href='/DocHtml/699/2014/1/8/6520459834636.html'>Unit 7 重难点归纳</a>")