When your computer begins to run slow, it is a ______ that it has been attacked by computer virus.      [ ]

A. signal    

B. symbol  

C. reality  

D. character

下列关于西北地区农业生产的叙述，正确的是 [ ]

A．农田主要分布在有河水、高山融雪水或地下水灌溉的地势平坦的地区

B．植被由东 向西呈现荒漠—荒漠草原—草原过渡的趋势

C．河西走廊和天山山麓一带广种水稻、小麦、棉花、瓜果

D．西北地区依靠冰雪融水，畜牧业和林业发达