Answer: One another & A reciprocal pronoun is used because two subjects of the sentence are the objects of the verb.
The compound pronouns one another or each other are reciprocal pronouns, and they are used to express a two-way interaction, that is, a reciprocal relationship: Kristin promised to send letters to Josh, and Josh promised to send letters to Kristin. Both do the action (subjects) and both receive the action (objects)
