Item A, two-way instruction means that the customer sends two instructions to the brokerage firm, and after one instruction is executed, the other instruction is automatically revoked;

Item b, stop the limit order refers to the order to execute the limit order when the market price reaches the trigger price set by the customer in advance;

Item D, Step Price Order refers to the order to gradually buy or sell a specified number of futures contracts within a specified price range. With this order, the price drops step by step when buying, and rises step by step when selling.

Therefore, this question chooses C.