In the past five years, there have been 4 derailment accidents of the xx high-speed train. In response to public questions about the design flaws of the xx high-speed train, the train manufacturer of the model has rebutted: the investigation showed that each derailment accident was due to driving Caused by a violation of operating procedures. Which of the following assumptions is the train manufacturer's refutation based on? ['When questioning the design flaws of the xx high-speed train, the public did not clearly indicate what kind of flaws existed.', 'Derailment accidents of high-speed trains in the past five years have not been caused by driver violation of operating procedures.', 'Investigators have the ability to find out whether the derailment accident was due to a flaw in train design or a problem in train manufacturing.', 'xx high-speed trains do not have any design defects that can cause drivers to violate operating regulations.']