关于现在完成式的疑问

问个苯问题，语法基础不好，请ｍｍ好心的回答，谢谢！！

以下是OG对现在完成式的解释，我的印象中完成式不是只有动作持续至现在，怎么又说完成于过去？？２１题是说该动持续至现在；另一个却说以完成于过去（这样不是没有持续了吗？）

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

Verb form +Logical predication

The simple past tense，used to describe an action completed in the past，is required in speaking of Native American migration to the Western Hemisphere．

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

B  The verb were migrating improperly describes a continuing action in the past；the adverb once should precede the verb existed．

C  The modifying phrase that once existed logically refers to the bridge of land，not the Western Hemisphere，which fortunately for many continues to exist．This phrase must be placed immediately following the word or word group that it modifies．

D  Correct．This sentence uses the correct verb tense，migrated, to describe an action completed in the past；the order of the prepositional phrases makes the meaning clear and 1ogical．

E  The verb were migrating is incorrect，and the phrase existing once imprecise.

The correct answer is D．

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

Agreement

The singular subject of the first independent clause is the period；the singular subject of the second independent clause is what is much more difficult to determine．These singular subjects require the singular verbs has and is．In this sentence，the second verb，are，is incorrect．

A  Are does not agree with what is much more difficult to determine．

B  Correct．In this sentence，both verbs are singular and therefore correct．

C  Have been established does not agree with the period．

D  Have been established does not agree with the period；are does not agree with what is much more difficult to determine．

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．

The correct answer is B．

请指正

大概了解ｍｍ的意思是也就是要看原本动作的特性，就所举例的”建立”，先看他是否是完成于过去，这表示该动作不会一直重复做下去，如果是完成于过去，再看他的结果是否持续到现在

谢谢mm的指导！

真是的，每次遇到时态的题目都不知道要如何下手！！