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A) The emergence of a new fast food chain has caused huge losses for XYZ Chicken in the same building last month.
B) The emergence of a new fast food chain caused huge losses for XYZ Chicken in the same building last month.

Which one is correct? If both are OK, which one is preferred?

My old high school grammar says that "last month" usually doesn't go with present perfect tense.
But I saw and heard many times their being used together in a sentence.

Is my high school grammar merely wrong or am i missing something?

Thanks guys,
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My try:
Bizncs A) The emergence of a new fast food chain has caused huge losses for XYZ Chicken in the same building last month.
B) The emergence of a new fast food chain caused huge losses for XYZ Chicken in the same building last month. If you say "last month" in the end of the sentence, it will be proper to use the simple past tense.

My old high school grammar says that "last month" usually doesn't go with present perfect tense.
But I saw and heard many times their being used together in a sentence.---- Well, you can't prescribe people how to choose their words.

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You say that you have heard "last month" used many times with the present perfect. In order for us to comment on any of those usages, you would have to post one/some of those sentences you've heard.

You should also keep in mind that saying 'in the last month' can easily be used with the present perfect -- even though 'in the last month' includes the words 'last month':

- I've seen him four times in the last month.
- I saw him four times last month.
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Thanks alot.Emotion: smile
 Yankee's reply was promoted to an answer.
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Point taken! Thanks~.