Which of the following made it easy for the united states to supply its efforts in world war i?

If these were the given choices: 
A: It had already obtained loans to develop manufacturing plants for war-time goods
B: It had the ability to finance war efforts through previous tax bonds.
C: It had manufacturing plants that could be easily converted to produce war-time goods.
D: It had already developed manufacturing plants for war-time goods to sell to its allies.

My answer would be: 
D: It had already developed manufacturing plants for war-time goods to sell to its allies.

Though the United States practice neutrality, they were still able to provide supplies to their Allies. So, when they entered WWI, they already have the weapons needed to fight the war.