关于时态的问题，GWD17-Q32

soojin3215

Navigators have known for thousands of years that the ocean has variable currents, but it is only in the last half century that a reasonably clear picture has emerged of the patterns and causes of ocean currents.

A) a reasonably clear picture has emerged of the patterns and causes of ocean currents
B) a reasonably clear picture of the patterns of ocean currents and their causes have emerged
C) a reasonably clear picture emerged of ocean currents, their patterns and the causes of them
D) there have emerged a reasonably clear picture of the patterns of ocean currents and what caused them
E) there had emerged a reasonably clear picture of the patterns of ocean currents and their causes

答案是A，我的疑问是既然发生在“ in the last half century”，为什么要用完成时，不用过去时？

soojin3215

啊，终于明白了，谢谢，所以下面这句也是这个原因才用完成时的吗？


All Y chromosomes in existence today are descended from a single ancestor’s who is thought to have lived about 140,000 years ago.
A) a single ancestor’s who is thought to have lived
B) a single ancestor’s who was thought to live
C) a single ancestor whom it is thought had lived
D) the Y chromosome of a single ancestor who is thought to have lived
E) the Y chromosome of a single ancestor whom it was thought had lived

soojin3215

这个我知道，因为是强调句 it is ... that...  还原就是
in the last half century，a reasonably clear picture has emerged of the patterns and causes of ocean current

soojin3215

可不可以理解为in last century 用过去时in the last century用完成时？