The right answer is Letter "D) ethnic tensions within the Austria Empire"
Explanation: The immediate cause pointed out, as responsible for the first world war. is the assassination, in Sarajevo, of the Archduke of the Austro-Hungarian Empire, Francisco Fernando, by the Serbian nationalist Gavrillo Princip. However, several other events contributed to the beginning of the conflict.
The answer is Ob The US government wanted to keep the involvement secret
D is the answer although it could be A. D or A.
Soviet Union is the answer