Using Pronoun Number and Gender
The word that a pronoun replaces is called its antecedent. If the antecedent is singular, the pronoun must be singular. If the antecedent is plural, the pronoun must be plural. They must agree in number.
The four friends couldn't believe their eyes: the Emerald City!
(Because the noun "friends" is plural, the pronoun "their" must be plural.)
Dorothy hoped the Wizard would grant her wish.