d.

</PRE>
<A name="References"><B><FONT SIZE="+1">References</FONT></B></A>
<PRE>
  [1] Enns, D.
      Model reduction with balanced realizations: An error bound
      and a frequency weighted generalization.
      Proc. CDC, Las Vegas, pp. 127-132, 1984.

  [2] Lin, C.-A. and Chiu, T.-Y.
      Model reduction via frequency-weighted balanced realization.
      Control Theory and Advanced Technology, vol. 8,
      pp. 341-351, 1992.

  [3] Sreeram, V., Anderson, B.D.O and Madievski, A.G.
      New results o