问题
                    
________we went to visit Professor Smith,he________his research work.
                            A:for
B:Whenever wasconcentrate on
C:For the firsttime was busy with
D:Each timewas absorbed in
                        
                        B:Whenever wasconcentrate on
C:For the firsttime was busy with
D:Each timewas absorbed in
正确答案是:
                        