Concept:The subject of a sentence must use subject pronouns like “I”, not object pronouns like “me”.Explanation:The blank is the subject of the sentence because it performs the action of the verb “were”.“I” is a subject pronoun, while “me” is an object pronoun and cannot be used as the subject.In English, when mentioning yourself with another person, it is natural and polite to put the other person first.Therefore, “his sister and I” is the correct phrase for the subject position.Answer:A. his sister and I