During the Great Financial Crisis, all the mentioned factors contributed to the U.S. dollar shortage:

• I. Increased appetite for US dollar-denominated assets: European banks actively accumulated U.S. dollar-denominated assets, notably mortgage-backed securities (MBS) and other structured products.
• II. Cross currency funding: To fund these investments, foreign banks engaged in cross-currency funding, borrowing in their local currencies and using foreign exchange swaps to convert them into U.S. dollars.
• III. U.S. dollar funding gap: As these funding sources were typically short-term, a massive U.S. dollar funding gap emerged when wholesale funding markets froze, leaving these banks unable to roll over their short-term dollar liabilities.

Therefore, I, II, and III are all correct causes of the U.S. dollar shortage.