Spot arbitrage of foreign exchange futures refers to the simultaneous reverse trading of foreign exchange spot and futures, that is, by selling overvalued foreign exchange futures contracts or spot. At the same time, buy undervalued foreign exchange futures contracts or spot to achieve the purpose of profit. Item a is correct. The transaction cost of foreign exchange futures mainly refers to the commissions charged by exchanges and commission dealers, as well as the transfer fees and other expenses incurred by the Central Clearing Company in buying and selling futures. Item b is correct. Exchanges or commissions will give different discounts, and the transaction costs of different investors will be different. Item c is correct. When calculating the no-arbitrage interval, the upper limit should be obtained by adding the theoretical price of futures to the transaction cost and impact cost of futures and spot; The lower limit should be obtained by subtracting the transaction cost and impact cost of futures and spot from the theoretical price of futures, and item D is correct. So the answer to this question is ABCD.