Which item is an example of secondary source

Which event to improve foreign relations between the united states and china? a. president truman's announcement of u.s. involvement in the "police action" in south korea in 1950 b. president carter's recognition of taiwan in 1979 c. president nixon's escalation of the vietnam war in laos and cambodia in 1973 d. president clinton's proposal to allow china to join the world trade organization (wto) in 1995