27. Diesel engines of conventional design fall into either a low-speed category or the medium and high-speed category. Low speed engines are generally intended for the direct drive of ship’s propellers without any speed-changing device and are restricted to an rpm range for which efficient propellers can be designed. Based on this description, which statement is correct? (A) A low-speed engine has to consider its speed range in order to match with the propeller design. (B) The design of efficient propellers is not related with the speed range of low-speed diesel engines. (C) The speed-changing device is required for a low-speed engine. (D) The low-speed engine could always provide efficient propulsion without consideration of its speed range.