PREP 138 时态问题

138. (32216-!-item-!-188;#058&006031)

According to scientists at the University of Alaska, while the surface temperature of the globe has

risen over the last century by about one degree Fahrenheit, the surface temperature in Alaska,

Siberia, and northwestern Canada has increased in the previous thirty years by about five degrees.

(A) the surface temperature in Alaska, Siberia, and northwestern Canada has increased in the

previous thirty years by about five degrees

(B) the surface temperature in Alaska, Siberia, and northwestern Canada has increased over the

last thirty years by about five degrees

(C) there was an increase in the last thirty years by about five degrees in Alaska, Siberia, and

northwestern Canada

(D) it had increased in Alaska, Siberia, and northwestern Canada by about five degrees over the

previous thirty years

(E) it has increased in Alaska, Siberia, and northwestern Canada over the previous thirty years by

about five degrees

答案是B

A错误的解释是in the previous 30 years 用过去式。。。。。。一个段的时间不是应该用完成时吗，有点糊涂了，求指教

现在完成时：since ，大段时间概念 last century ，past decade，nearly a century

XDF笔记上讲的，，，谁来给我讲讲。。

是不是previous 30 years 用现在完成

in previous 30 years用过去式啊。。。