Identify the correct sentence. a. someone opened the window because they be too warm. b. someone opened the window because them was too warm. c. someone opened the window because he or she was too warm. d. someone opened the window because they were too warm.

C. Someone opened the window because he or she was too warm.

Explanation:

The more recent grammatical usage marking the explicit inclusion of the feminine in the use of  pronouns, not the old implicit inclusion where we always assumed the masculine included the feminine.

So, we need to use "he or she" to match with the number expressed in the verb (singular).  If we would use 'they', that pronoun is plural.  The answer D would have been correct in the past, not nowadays.

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explanation: