The word "only" is a clue that this answer may be incorrect. If one of the compounds is an acid, it is more likely for the proton to react with an anion to form polar-covalent bonds. This alone is not sufficient to drive the reaction unless the product that is formed is not soluble in the aqueous solution. If this product forms a gas, then the loss of the gas would drive the reaction. An example of this would be the reaction of the hydrogen carbonate anion with a Brønsted acid. Is there any other way that this type of reaction could occur?