What might have made the Rosenbergs seem dangerous to the US government of the 1950s?

The Rosenbergs were associated with Communist and other left-wing organizations, which fueled suspicion during the intense anti-communism of the 1950s. That political association could make them seem dangerous to the U.S. government. The other choices are inaccurate: they did not both create the Soviet atomic bomb, Ethel was born in the United States, and Julius did not work for the VENONA project.

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