同一页面的FrameA如何控制FrameB包含页面里的对象？

C.HTM
<FRAMESET ROWS="," COLS="50,*">
<FRAME SRC="a.htm" NAME="a">
<FRAME SRC="b.htm" NAME="b">
</FRAMESET>a.htm
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<TITLE> New Document </TITLE>
<META NAME="Generator" CONTENT="EditPlus">
<META NAME="Author" CONTENT="">
<META NAME="Keywords" CONTENT="">
<META NAME="Description" CONTENT="">
</HEAD><BODY></BODY>
</HTML>b.htm
<SCRIPT LANGUAGE="JavaScript">
<!--
parent.a.document.body.innerHTML='OK';;
//-->
</SCRIPT>