What led the us and their allies to get involved the vietnam war

Concerns about Communism spreading if Vietnam went communist, this was known as Domino Theory. U.S. foreign policy was shaped by Anti-communist views.

The  United States and its allies worried that if Vietnam fell to the communists, other countries in Southeast Asia would follow.  This was known as "domino theory" -- that the spread of communism in Vietnam would result in other countries in the region falling like dominoes to the influence of communism.  

President Dwight Eisenhower is the one who initiated the use of the term "domino theory."  In a speech he gave in April, 1954, Eisenhower warned that if French Indochina fell to communism, it very likely could cause a domino  effect in Southeast Asia.  French Indochina is what the region of Vietnam was called during the days of French colonialism there.  The movement against French colonial control was led by Vietnamese nationalist who also happened to be communists.  The Vietamese nationalist / communist movement was led by HoChi Minh.

the underground railroad (apocope of the metropolitan railroad) is a system of urban trains located within a city and its metropolitan area. it is characterized for being a massive transport of passengers in the big cities, uniting diverse zones and their environs, with high capacity and frequency and in segregated form to other systems of transport. metropolitan railroads are built underground or on the surface, although most systems use mixed models where sections are combined in both modalities.

1 - as a result, the metro tends to reduce car travel for two reasons: first, due to the greater transfer of users from the car and, second, due to the reduction in the acquisition of cars in homes located in sectors near the stations of the metro.

2 -the metro allows to contribute effectively to the achievement of all the objectives of an urban development policy: it improves the efficiency of the city's economy by reducing travel costs and also generates a higher level of activities in the city center, taking advantage of of the agglomeration economies.

3 -it is a clean means of transport, since it uses renewable energy (usually electricity), taking care of the environment and avoiding excessive pollution.

the answer to your question is a. the salt plains

in 1811, he led an expedition, known as the george c. sibley expedition, to improve relations with the pawnee and kansa tribes, and also to locate the rumored jefferson's salt mountain