The main purpose of the selective service act of 1917 was to
increase the size of the armed forces by using the draft.
restrict membership in the armed forces to those with prior combat experience.
allow all branches of the armed forces to be sent to europe.
equip the armed forces with the necessary weaponry for battle.

The right answer is increase the size of the armed forces by using the draft.

In the heat of the WWI and the decision to enter the war, the USA had only a combined strength of the regular army and National Guard of only 379,000 combatants. The need for a large number of troops forced Wilson to implement a military draft called Selective Service Act of 1917, where all men aged twenty-one to thirty (later, eighteen to forty-five) could be drafted for military service. About 2 million mostly under-trained American troops crossed the Atlantic, and about 1.4 million saw at least some combat, including 42,000 African Americans. In the end, all the USA combined strength total of 3.7 million.

an ethnic enclave is a geographic area with high ethnic concentration,

(d) rivers.

explanation:

civilizations developed around rivers because their waters provided places to hunt and fish. also, as the rivers flooded, the lands around them became fertile. this allowed them to support farming. this is especially true of the nile river, which flooded the same time each year.