We thank the editor Dr. Alvin Bayliss and three anonymous reviewers for constructive criticism of our paper. Liu would like to thank China Scholarship Council for their financial support for this research and UTIG for providing with the facilities. This research is also partially supported by NSFC under Contract No. 40839901, the National “863” Program of China under Contract No. 2007AA06Z218 and KAUST AEA grant at UT Austin.