What prompted China to enter the war directly?

the UN victory at Inchon
the early successes of North Korea
the US threat to use nuclear weapons
the UN victories in the northern peninsula

What prompted China to enter the war directly.

Explanation:

1.UN forces invaded North Korea in October 1950 and moved rapidly towards the Yalu River—the border with China—but on 19 October 1950, Chinese forces of the People's Volunteer Army (PVA) crossed the Yalu and entered the war.

2.The 300,000-man Chinese offensive caught the U.N. forces off guard, largely because of U.S. Gen. Douglas MacArthur's belief that China would not openly enter the war, and vastly expanded the conflict. The Korean War began when communist North Korean forces invaded democratic South Korea on June 25, 1950.

3.A year after the creation of the PRC, Mao decided that China would enter the Korean War and fight the United States and its United Nations allies for control of the Korean peninsula. The war in Korea had begun on June 25, 1950, when communist North Korea invaded the South