The immediate cause of the united states entry into world war i in 1917 was

The resumption of unrestricted submarine warfare by Germany, from 31 January 1917. 
German u-boats were sinking US merchant ships and killing US seamen even though USA was a neutral. Further, ships of other neutral nations sailing to and from the US were also being sunk.All this was adversely effecting the US economy.
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The answer would be option a , b, and c.