Marinkov, N.
Markova-Velichkova, M.
Gyurov, S.
Kostova, Y.
Spassova, I.
Rabadjieva, D.
Kovacheva, D.
Penkov, I.
Tzvetkova, Ch.
Gentscheva, G.
Article History
Received: 20 March 2018
Accepted: 5 July 2018
First Online: 19 July 2018
Compliance with ethical standards
:
: The authors declare that they have no conflict of interest.