Which of the following is not a factor that contributed to the collapse of the soviet union

Factors which led to collapsing of Soviet Union may include;
(i)   Romanian revolution. The Berlin which was made as a barrier by the German Democratic Republic in 1961 with hopes that it will keep East canals away from escaping to the West sides. Berlin wall was termed as a symbol of being stable and prevented East from contacting with the West.
(ii)   Solidarity. This movement started because communist Poland wanted politics which are reformed.One protest was made together by fifty labour Unions. The movement acted as a catalyst which allowed Poland to become the democracy member of EU from being repressed under communist rule.
(iii)  Velvet wall. This was a non-violent revolution which was used to protest against communist party.
(iv)  Prague spring. This was started by reformist Dubek Alexander. Members invaded the country to end reforms.