The key factor in the decision of the U.S. to enter World War I was "<span>d. Germany's resumption of unrestricted submarine warfare against the U.S.," since this led to the sinking of the Lusitania, which had many US citizens on board. </span>
According to the passage, unequal distribution of resources leads to "<span>B. specialization and trade" since those who lack certain goods trade the goods they do have, so everyone "wins". </span>
