Answer :

Answer;
Iron Curtain;
When Hungary opened its borders, it created a "hole" in the iron curtain.

Explanation;
Hungary was the first country to punch a hole in the Iron Curtain.
This meant that many refugees from Eastern Germany who had fled to Hungary, were going to leave and move to Western Germany. They would take this trip by going through Austria (neighboring country of Hungary) to reach Western Germany. Mikhail Gorbachev was personally outraged by this, stating that Hungary had not asked him before taking this measure.

The correct answer to this open question is the following.

When Hungary opened its borders, it created a "hole" in the iron curtain.

Hungary was the first country to open its borders on September 11, 1989. This represented a magnificent opportunity for people to leave the socialist countries under the influence of the Soviet Union. That is why historians coined the term "it created a "hole" in the iron curtain." When this happened, thousands of people that lived in East Germany left and went to the Western Europe countries. Socialism had collapsed in Hungary.