Can any one tell explanation for below program answer

Question

using System;
class program
{
static void Main(string[] args)
{
int[] arr = new int[] {1 ,2 ,3 ,4 ,5 };
fun1(ref arr);
Console.ReadLine();
}
static void fun1(ref int[] array)
{
for (int i = 0; i < array.Length; i=i+2)
{
array[i] = array[i] + 10;
}
Console.WriteLine(string.Join(",", array));
}
}

Ans: 11,2,13,4,15