Which of the following is true about scientific theories? i. new research is often done to learn more about existing scientific theories. ii. scientific theories can be revised, changed, or even rejected. iii. scientific theories are considered to be facts and cannot ever be modified. iv. new scientific theories can be formed to explain new phenomena.

i believe the correct answer from the choices listed above is the first option. the scientists need to look at the   validity of data, claims, hypotheses, and observations before  developing an argument. they base their arguments on facts that are present and they make sure they have proofs. 

explanation:

the truman doctrine was an american foreign policy whose stated purpose was to counter soviet geopolitical expansion during the cold war. it was announced to congress by president harry s. truman on march 12, 1947, and further developed on july 12, 1948, when he pledged to contain threats to greece and turkey.