The south vietnamese invaded with american air support.
a. thailand
b. laos
c. cambodia

The correct answer is B, as the South Vietnamese invaded Laos with American air support, in the Operation Lam Son 719.

Operation Lam Son 719 was a military offensive developed by the South Vietnamese Army led by General Xuan Lam in command of 20,000 soldiers and with US air support that left the city of Khe Sanh in 1971, during the Vietnam War.  This mission was trying to cut by land the HoChi Minh Route that contributed important supplies and troops to the contingents that fought in the South.

The action, which involved an invasion of Laos by a breach of 35 km long and 25 km wide, failed in its attempt to suppress supplies and reinforcements sent from the north to the south of Vietnam through Laos.

relations between the soviet union and the united states were driven by a complex interplay of ideological, political, and economic factors, which led to shifts between cautious cooperation and often bitter superpower rivalry over the years. the distinct differences in the political systems of the two countries often prevented them from reaching a mutual understanding on key policy issues and even, as in the case of the cuban missile crisis, brought them to the brink of war.

the united states government was initially hostile to the soviet leaders for taking russia out of world war i and was opposed to a state ideologically based on communism. although the united states embarked on a famine relief program in the soviet union in the early 1920s and american businessmen established commercial ties there during the period of the new economic policy (1921–29), the two countries did not establish diplomatic relations until 1933. by that time, the totalitarian nature of joseph stalin's regime presented an insurmountable obstacle to friendly relations with the west. although world war ii brought the two countries into alliance, based on the common aim of defeating nazi germany, the soviet union's aggressive, antidemocratic policy toward eastern europe had created tensions even before the war ended.