OG verbal 21&24 關於現在完成式的用法

[chiehwu]

21． It is possible that Native Americans originally have migrated to the Western Hemisphere over a bridge of land that once existed between Siberia and Alaska.
(A) have migrated to the Western Hemisphere over a bridge of land that once existed
(B) were migrating to the Western Hemisphere over a bridge of land that existed once
(C) migrated over a bridge of land to the Western Hemisphere that once existed
(D) migrated to the Western Hemisphere over a bridge of land that once existed
(E) were migrating to the Western Hemisphere over a bridge of land existing once

A The present-perfect verb have migrated implies that the migration is continuing into the present．

24．The period when the great painted caves at Lascaux and Altamira were occupied by Upper Paleolithic people has been established by carbon-14 dating，but what is much more difficult to determine are the reason for their decoration，the use to which primitive people put the caves，and the meaning of the magnificently depicted animals．
(A) has been established by carbon-14 dating，but what is much more difficult to determine are
(B) has been established by carbon-14 dating，but what is much more difficult to determine is
(C) have been established by carbon-14 dating，but what is much more difficult to determine is
(D) have been established by carbon-14 dating，but what is much more difficult to determine are
(E) are established by carbon-14 dating，but that which is much more difficult to determine is

E Are established does not agree with the period；this verb should be in the present perfect tense to express action completed in the past；that which construction is ungrammatical．

我有問題的是在答案針對完成式解釋的部份. 21題說現在完成式是暗示了動作持續到現在, 24題則說現在完成式動做在過去已經完成. 這兩個是同時成立的嗎? 有點困惑.

答案沒有問題, 只是我在歸納用法的時候有點疑惑..

[dibert8]

現在完成式表示過去的動作現在為真.
21. migrate 現在已不復存在
24. 現在仍為事實