A nightingale is a bird known for its beautiful, melodious song. The phrase "that of the" specifies that the voice is being compared to the sweetness of a nightingale's song. Therefore, the complete and grammatically correct sentence is "Her voice is as sweet as that of the nightingale."
Her voice is as sweet as ______ nightingale.
Review the verified answer and explanation for this Audit past-paper question on CampusFin.