Authors
R. Georgieva-Nikolova
Bulgaria
H. Hristov
Bulgaria
M. Nikolova
United Kingdom of Great Britain and Northern Ireland
© 2017 M. Slavova, R. Georgieva-Nikolova, H. Hristov, M. Nikolova, published by Konstantin Preslavski University of Shumen
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License.