Why doesn't LINQ work properly?

Question

.NET

I just started learning LINQ and sometimes there are difficulties,
why does it output only "sa" and not a string from the first characters of the array of strings?

//4. Дана последовательность непустых строк.Используя метод Aggregate,
            // получить строку, состоящую из на-
            //чальных символов всех строк исходной последовательности. 

            string[] s = new[] { "sgsg", "tdfgsdfg", "rdfgsdfg", "iiutyiu", "4fghdhsfd", "kasdgfdsf", "asdfdfsd" };

            var res = s.Aggregate((a,b) => a.First() + "" + b.First() );

            Console.WriteLine($"{res}");

            Console.ReadLine();

Answer 1

string[] s = new[] { "sgsg", "tdfgsdfg", "rdfgsdfg", "iiutyiu", "4fghdhsfd", "kasdgfdsf", "asdfdfsd" };

var res = "";

res = s.Aggregate(res, (a, b) => a + b[0]);

Console.WriteLine($"{res}");

Console.ReadLine();