Answer:
The answer for this Question is D
Explanation:
The railroad decreased transportation time across the United States from about six months to roughly "one week," which was incredibly important in terms of settlement and business in the west. <span><span>
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After the Soviets successfully detonated an atomic bomb in 1949, President Harry S. Truman ordered the creation of a hydrogen bomb project. With the help of the Polish mathematician Stanislaw Ulam, Dr. Teller developed a model for the hydrogen bomb known as the Teller-Ulam design