which river's source mountain is zagonul mare river and it's mouth is covasna river ?
SELECT DISTINCT var_uri where brack_open var_uri dbo_sourceMountain dbr_Zagonul_Mare_River sep_dot var_uri dbo_riverMouth dbr_Covasna_River_ attr_open Râul_Negru attr_close sep_dot var_uri a dbo_River brack_close