Which of the following CANNOT be used as an underlying datatype for an enum in C#.NET?

1. A function cannot be called from a subroutine.
2. The ref keyword causes arguments to be passed by reference.
3. While using ref keyword any changes made to the parameter in the method will be reflected in that variable when control passes back to the calling method.
4. A subroutine cannot be called from a function.
5. Functions and subroutines can be called recursively.

decimal static fun(int i, Single j, double k)
{ ... }

decimal fun(int i, Single j, double k)
{ ... }

static decimal fun(int i, Single j, double k)
{ ... }

static decimal fun(int i, Single j, double k) decimal
{ ... }

static fun(int i, Single j, double k)
{ 
    ... 
    return decimal;
}

class Sample
{
    public void MyFun(int i, Single j)
    {
        Console.WriteLine("Welcome to CuriousTab !");
    }
}

delegate void del(int i);
Sample s = new Sample();
del d = new del(ref s.MyFun);
d(10, 1.1f);

delegate void del(int i, Single j);
del d;
Sample s = new Sample();
d = new del(ref s.MyFun);
d(10, 1.1f);

Sample s = new Sample();
delegate void d = new del(ref MyFun);
d(10, 1.1f);

delegate void del(int i, Single]);
Sample s = new Sample();
del = new delegate(ref MyFun);
del(10, 1.1f);

namespace CuriousTabConsoleApplication
{
    class SampleProgram
    {
        static void Main(string[ ] args)
        {
            object[] o = new object[] {"1", 4.0, "India", 'B'};
            fun (o);
        }
        static void fun (params object[] obj)
        {
            for (int i = 0; i < obj.Length-1; i++)
            Console.Write(obj[i] + " ");
        }
    }
}

    int[] intMyArr = {25, 30, 45, 15, 60};

1. intMyArr.GetMax;
2. intMyArr.Highest(0);
3. intMyArr.GetUpperBound(0);
4. intMyArr.Length;
5. intMyArr.GetMaxElements(0);

namespace CuriousTabConsoleApplication
{
    class SampleProgram
    {
        static void Main(string[ ] args)
        {
            int i;
            int res = fun(out i);
            Console.WriteLine(res);
        }
        static int fun (out int i)
        {
            int s = 1;
            i = 7;
            for(int j = 1; j <= i; j++)
            {
                s = s * j;
            }
            return s;
        } 
    } 
}