First Author:   Tuan Nguyen Dalat University tuannv@dlu.edu.vn   01 Phu Dong Thien Vuong Street Dalat, +84, Vietnam   1218 280 442(Phone) 1218 280 442(Fax) Second Author: An Nguyen Dalat University annth@dlu.edu.vn   01 Phu Dong Thien Vuong Street Dalat, +84 263, Vietnam   3831105(Phone)