The answer is C
It was made illegal because slave owners feared that their slaves wouldn’t depend on their owners
Own a slave in The US, and buy or sell slaves.
The Flexible response was a policy implemented by President John F. Kennedy in 1961 in order to substitute the New Look and the massive retaliation policy that Dwight Eisenhower's had introduced, which consisted on responding using a greater force in case of an attack, and such force involved nuclear weapons.
On the other hand, the flexible response policy aknowledges the Mutual assured destruction if nuclear weapons are involved, as the enemy with shoot back too. It aimed to provide a manner of responding to agression across the spectrum of war but without the employment of nuclear weapons.
The train was the greatest impact i believe