问题
                    
It’s difficult to ___ these old books for some of them ______ none of the classification.
                            A:look for; are belonging to
B:put up; belong
C:sort out; belong to
D:pick up; are belonged to
                        
                        B:put up; belong
C:sort out; belong to
D:pick up; are belonged to
正确答案是:
                        