This task is about reading the given passage and construct a question about the information present in the passage. Construct a question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique (iv) its answer is a continuous text span from the paragraph. Avoid creating questions that (i) can be answered correctly without actually understanding the paragraph and (ii) uses same words or phrases given in the passage.

Since the early 1990s, Estonia is involved in active trilateral Baltic states co-operation with Latvia and Lithuania, and Nordic-Baltic co-operation with the Nordic countries. The Baltic Council is the joint forum of the interparliamentary Baltic Assembly (BA) and the intergovernmental Baltic Council of Ministers (BCM). Nordic-Baltic Eight (NB-8) is the joint co-operation of the governments of Denmark, Estonia, Finland, Iceland, Latvia, Lithuania, Norway and Sweden. Nordic-Baltic Six (NB-6), comprising Nordic-Baltic countries that are European Union member states, is a framework for meetings on EU related issues. Parliamentary co-operation between the Baltic Assembly and Nordic Council began in 1989. Annual summits take place, and in addition meetings are organised on all possible levels: speakers, presidiums, commissions, and individual members. The Nordic Council of Ministers has an office in Tallinn with a subsidiary in Tartu and information points in Narva, Valga and Pärnu. Joint Nordic-Baltic projects include the education programme Nordplus and mobility programmes for business and industry and for public administration.
Which Baltic countries have cooperated with Estonia since the early 1990s?