问题
                    
His parents are convinced ____ the fact the their son always focuses his mind ___ his study
                            A:of; on
B:of; at
C:by; to
D:on; at
                        
                        B:of; at
C:by; to
D:on; at
正确答案是:
                         
                        
                        
                    同类题型
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