Which of these was a factor in the United States entrance in World War I? A) the invasion of Poland B) Japan's invasion of China C) The Axis threat to the Panama Canal D) German unrestricted submarine warfare

D) German Unrestricted submarine warfare 

~Hello there! ^_^ Your question: Which of these was a factor in the United States entrance in World War I? Your answer: German unrestricted submarine warfare was a factor in the United States entrance in World War I. The answer is option D. Hope this helps~