You will be given a trivia clue, and the category it belongs to. You should answer with the best answer that belongs in the category and is described by the clue. For consistency, answers should be in all lower cased letters.

Category: IT'S NOT SO 
Clue: Though 4,500 miles long, it's not visible from space, as its materials are similar in color to the surrounding soil
the great wall of china

Category: THE 16th CENTURY 
Clue: In the 1569 Union of Lublin, Zygmunt II united Lithuania & this country
poland

Category: MOTTOES 
Clue: Strangely, "Blood & fire" is the motto of this Christian charitable organization

the salvation army