the last half of the nineteenth century ____ the improvement in the means of travel

题目

the last half of the nineteenth century ____ the improvement in the means of travel
a.has witnessed
b.witnessed
为什么选b不选a 为什么不可以用现在完成时

答案

这是陈述过去事实,与现在无时间关联,这从主语the last half of the nineteenth century “19世纪后半叶” 可以看出