The authors are grateful for the comments and suggestions from the reviewers. This work was supported by the Chinese NSF Grant 40275012, the National Key Basic Research Project G1998040907, and Grant OCE9986368 (with a supplement from the International Programs Office) from the U. S. NSF. The main part of the work was done when the first author, B. Tan, was visiting the AOSS department at the University of Michigan. He is grateful to the support from the department.
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