What was a common thread in the fall of communist regimes throughout eastern europe

The peaceful collapse of the regimes was by no means pre-ordained. Soviet tanks crushed demonstrators in East Berlin in June 1953, in Hungary in 1956, and again in Czechoslovakia in 1968. Soviet military planners were intimately involved in the Polish planning for martial law in 1980, and Soviet troops remained stationed throughout Eastern Europe, as much a guarantee for Soviet security as an ominous reminder to Eastern European peoples of Soviet dominance over their countries.

So whats the answer 
a. a decline in education and health care 
b. poor economies and demands for more rights 
c. a lack of strong leaders in communist governments 
d. support fro military alliance with the United States