المصدر: :253-258
URL الوصول: http://elibrary.worldbank.org/doi/pdf/10.1596/978-0-8213-9715-2http://elibrary.worldbank.org/doi/abs/10.1596/9780821397152_App-Nhttp://elibrary.worldbank.org/doi/pdf/10.1596/9780821397152_App-Nhttp://elibrary.worldbank.org/doi/abs/10.1596/978-0-8213-9715-2