Read the article and select the best answer. Article: WASHINGTON -- President Obama brought together the leaders of Japan and South Korea to discuss cooperation on containing North Korea and its nuclear weapons programs. Japan and North Korea have had tense relations over unresolved issues, some of them dating back 100 years. The three leaders emphasized the importance of cooperation in dealing with North Korea. The meeting took place Tuesday in the Hague, after a twoday nuclear security summit. President Obama praised South Korean President Park Geunhye and Japanese Prime Minister Shinzo Abe for overcoming their differences to discuss a serious threat to regional peace. He emphasized the importance of cooperation on the North Korean nuclear issue. Obama said the three discussed steps to deepen diplomatic and military cooperation, including joint military exercises and missile defense against Pyongyang. He said further discussions would take place next month, during his visits to Seoul and Tokyo. The South Korean president stressed the importance of a united response to North Korea, but also called on the North to choose a peaceful path. "I sincerely hope that this meeting will offer a chance for us to reconfirm our trilateral coordination and strengthen the cooperation on the nuclear front," said Park. Japanese Prime Minister Shinzo Abe agreed. "Particularly it is extremely important that we were able to confirm close cooperation among Japan, the United States and the Republic of Korea on the issue of North Korea, the three countries who would like to cooperate so that North Korea will be able to take a positive position with regard to nuclear and missile issue," said Abe. Japan and South Korea have several unresolved issues, including Japan's refusal to apologize again for crimes committed 70 years ago. Former U.S. diplomat Richard Armitage told an audience this week that painful historic injustices take a long time to heal, and that it is in Japan's interest to continue apologizing for as long as necessary. "There is a historical record of enormous... Question: What is the main topic about the three leaders' meeting? Options: A: Diplomatic and military cooperation. B: A peaceful path to solve the Asia problems. C: A serious threat to regional peace. D: A united response to North Korea.
Answer:
D