using System;
using System.Linq;
class Program
{
static void Main() {
int[][] array2d = {
new[] {4, 5, 3},
new[] {30, 20},
new[] {10},
new[] {1, 2, 6, 7, 8},
};
var arr = array2d.SelectMany(x => x).ToArray();
Console.WriteLine(string.Join(", ", arr));
}
}
/*
run:
4, 5, 3, 30, 20, 10, 1, 2, 6, 7, 8
*/