Compulsory thinking, also known as thinking gathering, refers to a large number of thinking that does not belong to the patient's mind. These thinking are not controlled by the patient's will, and emerge compulsively in the brain. The content is mostly disordered, sometimes even disgusted by patients. According to the above definition, Which of the following is one of the mandatory thinking? ['A very slow thinking activities, difficulty in association, difficulty thinking, slow response.', 'Associative slack in the thinking activity, loose content, not focusing on the problem, not deep.', 'C often feels that his thinking is disturbed by external factors and he is very disgusted.', 'Ding often feels that he has some cluttered ideas in his head that he cannot control at all.']