There are no options given in this question, but i hope you wanted to know whether the statement in the question is true or false. It is completely false to state that the Constitution was completed in the year 1776. actually the Constitution of the United States of America was completed on 17th of September in the year 1787.
The correct answer to this open question is the following.
The major cause of the end of Reconstruction was the informal negotiations between Democrats and Republicans to declare the winner of the elections of teh 1876 Presidential election. They negotiated and Democrats agreed to declare Rutherford B. Hayes with the following conditions.
Republicans accepted to sign the Compromise of 1877, in which they agreed on withdrawing the federal troops established in the southern states that were there to support Reconstruction. So without the presence and support of the federal troops, Reconstruction saw its last days and finally ended in 1877.
Following the Seven Years War, the British Empire was able to secure all territories from the Atlantic coast to the Mississippi River. This was a great achievement for the Britsih Empire who now controlled larger portions of the continent at the cost of a French presence in North America. However, the presence of the English was not challenged, as a result of the Seven Years War, the Spanish also amassed vast territories on the continent.
