Which sentence demonstrates correct use of the past perfect tense?

A. She had finished work when the phone rings.

B. By 2020, they had already developed the prototype.

C. I have seen that movie yesterday.

D. He has been working here since 2015.

答案解析

Core concept: Past perfect tense usage (action completed before another past action). Correct answer: B. Shows completion before specific past time (2020). Error analysis: A mixes past perfect with present tense; C/D use present perfect incorrectly. Key marker: 'By + time' structure requiring past perfect.

正确答案：B

Which sentence demonstrates correct use of the past perfect tense?

答案解析

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