STRUCTURE OF THE THESIS The main part of the thesis is 115 pages long, consisting of the following sections: Introduction: 2 pages; Chapter 1 - Overview: 30 pages; Chapter 2 - Research Object and Method: 20 pages; 3 Chapter 3 - Research Results: 28 pages; Chapter 4 - Discussion: 29 pages; Conclusions and recommendations: 3 pages The thesis has 140 references, of which 24 are in Vietnamese and 116 are in English.