JSON response through .NET

I am working with .NET to fetch data from MongoDb.

When the below API method is called through the API

        public IActionResult FirstStudent()
         var collection = this.database.GetCollection<BsonDocument>("students");
            var filter = Builders<BsonDocument>.Filter.Eq("RollNo", "1");
            var document = collection.Find(filter).First();          
            var firstStudent= document.ToJson();
            return Ok(firstStudent);

the response has Content-Type as text/plain. I need the Content-Type as application/json.

Any suggestions?