In 1978, under the leadership of Deng Xiaoping, China initiated its "Reform and Opening Up" policy. This involved opening up the Chinese economy to foreign investment and trade, and implementing market-oriented reforms. This policy is often referred to as the "Open Door Policy."
Open Door Policy: A policy of allowing multiple foreign powers access to a country's markets and resources, generally avoiding exclusive control by any one power. In the context of 1978, it implies actively encouraging foreign investment and trade.
So, the correct answer is (B): China
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\(2, 6, 12, 20, 30, \ ?\)