Austria General Knowledge (Trivia) Questions

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1.
Which one of the following is the official and national language of Austria?
2.
The famous Danube River is the longest river of which one of the following nations?
3.
Which one of the following is the capital of Austria?
4.
The famous tourist destination "Salzburg Land" is located in which one of the following nations?
5.
In which one of the following nations "Eisriesenwelt" or "World of the Ice Giants", the largest ice cave in the world, is located?