Step 1: Understanding the events.
- The Berlin Blockade (1948-1949) was the first major crisis of the Cold War, where the Soviet Union blockaded West Berlin, leading to the Berlin Airlift.
- The Berlin Wall was constructed in 1961 to stop the flow of refugees from East Germany to West Germany during the Cold War.
- The Soviet invasion of Czechoslovakia occurred in 1968 when Soviet troops intervened to suppress the Prague Spring, a movement for political reforms.
- Communist China joined the UN in 1971, marking a significant shift in global politics.
Step 2: Chronological Order.
The correct chronological order is:
1. (D) Berlin Blockade (1948-1949)
2. (A) Berlin Wall is constructed (1961)
3. (C) Soviet invasion of Czechoslovakia (1968)
4. (B) Communist China joins the UN (1971)
Step 3: Conclusion.
The correct answer is (2) D, A, C, B, which correctly orders the events based on their occurrence in the Cold War period.