Web Api

Hi,

I have a question, im creating a web api to return data from a query and i have to remove a column from the response.

For example:

I have an expression rule to bring me all the centers we have and it returns  "id" : null, "address" : "null", "officeName" : "null", "countryId" : null, "isAvailable" : true

How can i remove the column "countryId"?