The reason:
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B) Mary cans dance tomorrow: "Cans" is not the correct form of the verb "can." The correct form is "can," which is used to express ability or possibility.
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C) Mary she can dance tomorrow: The pronoun "she" is redundant and incorrectly placed. In English, we don't need to repeat the subject when using modal verbs like "can."
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D) Mary can tomorrow dance: The placement of "tomorrow" is incorrect. In English, time expressions typically come at the end of the sentence or after the verb but before the main action.
In option A, the sentence correctly follows the standard word order for English sentences: subject (Mary) + modal verb (can) + main verb (dance) + time expression (tomorrow).