What event most directly led to construction of berlin wall?

At the end of the Second World War, Berlin was divided into four sectors of occupation: Soviet, French, English and American.

Subsequently, in 1949 the eastern area that was held by the Soviets was under the jurisdiction of the so-called German Democratic Republic (RDA) and the other three formed a single area governed by the Federal Republic of Germany (FRG).

In the following years, the western zone began to prosper while the Soviet side suffered economic problems. This, caused millions of Berliners who lived under the communist regime to migrate to West Berlin.

It is for this reason that on the night of August 12, 1961 the authorities of the GDR decided to build a provisional wall that divided the city in two.

your answer would be (a)

i dont know but i need points