听力原文： Vietnam's prime minister has announced his plans to visit the United States next month. He'll be the first Vietnamese leader to travel to the US since the end of the Vietnam War. Prime Minister Phan Van Khai made the announcement while on an official trip to Australia. The prime minister told reporters he's planning to visit the United States in June, so the two countries could improve their relationship. He indicated issues relating to China, Vietnam's northern neighbor wouldn't be raised during the trip. He also indicated the US visit didn't signal a change in Vietnam's relationship with China. Last month marked 30 years since the fall of Saigon, now known as Ho Chi Minh City, and the end of the Vietnam War. Over 58,000 Americans were killed in the Vietnam War or remain missing, presumed dead. Over one million Vietnamese soldiers died and many more civilians. When did Vietnam's prime minister announce his plan?