Hitler focused on the Rhineland and the sundentenland and primarily because

Pan-Germanism and revanchism

Explanation:

Hitler and the Nazi party's main goal was the creation of a Greater Germanic Reich, Which would encompass Germany proper, Denmark, Netherlands,Belgium,the Czech half of Czechoslovakia, Austria, Alsace-Lorraine, Nord-Pas-de-Calais, Sweden, Norway, and Finland. Poland and all of Russian Europe up to the Urals were also to become part of this Super-state Encompassing the continent through a period of resettlement and colonization. The militarization of the Rhineland, the Anchluss of Austria, and the annexation of the Sudetenland was but the first step to achieving Hitler's vision. Hope it helps!

c, they united with once

b is correct

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