The marshall plan was important because it

The Marshall Plan rebuilt Western Europe and secured allies for the United States.

Explanation/context:

The "Marshall Plan" was named after the man who then was US Secretary of State, George C. Marshall.  Officially the plan was called the European Recovery Program.  Marshall announced the plan in 1947, and it went into effect in 1948.  The intent was to provide aid and rebuilding to European economies after the damaging effects of World War II.   Eastern bloc countries, however, rejected the plan, so it ended up as a plan that benefited Western European nations and not Eastern European nations.

In his speech introducing the plan, Secretary Marshall had said:  "Our policy is not directed against any country, but against hunger, poverty, desperation and chaos. Any government that is willing to assist in recovery will find full co-operation on the part of the United States. Its purpose should be the revival of a working economy in the world so as to permit the emergence of political and social conditions in which free institutions can exist."

The view in the communist-controlled Eastern bloc was that the US was trying to use such a policy to spread its influence and threaten their patterns of government under communism.  So the plan ended up building allies for the US in Western Europe, while the Eastern European countries sided with the Soviet Union.

b) race riots in several cities

the name of the theory is landmass theory.