19. however, if not managed properly, ______________ can create many problems. sometimes ______________. furthermore, ______________. therefore, ______________ has been gaining public concern.
        20. there are probably many reasons for ______________. first, ______________. second, ______________. finally, ______________.
        21. well, why is there ______________? i think there might be two reasons. one is ______________, and the other is ______________.
        22. there are, i think, two main reasons for ______________. in the first place, ______________. in the second place, ______________. therefore, ______________.
        23. why ______________? for one thing, ______________. for another, ______________. perhaps the main reason is ______________.
        24. it is no easy job to find the reasons for this tendency which involves several complicated factors. for some ______________. for others ______________.
        25. why ______________? the first reason is that ______________. the second reason is ______________. the third is ______________. for all this, the main cause of ______________ is due to ______________.
        26. it is reasonable to maintain that ______________, but it would be foolish to persist that ______________. there are three reasons ______________.