Which of the following statements are correct about a namespace used in C#.NET?

1. Classes must belong to a namespace, whereas structures need not.
2. Every class, struct, enum, delegate and interlace has to belong to some or the other namespace.
3. All elements of the namespace have to belong to one file.
4. If not mentioned, a namespace takes the name of the current project.
5. The namespace should be imported to be able to use the elements in it.

byte b1 = 0xAB;
byte b2 = 0x99;
byte temp;
temp = (byte)~b2;
Console.Write(temp + " ");
temp = (byte)(b1 << b2);
Console.Write (temp + " ");
temp = (byte) (b2 >> 2);
Console.WriteLine(temp);

int val;
for (val = -5; val <= 5; val++)
{
    switch (val)
    {
        case 0:
            Console.Write ("India"); 
            break;
    }
    
    if (val > 0)
        Console.Write ("B"); 
    else if (val < 0)
        Console.Write ("X");
}

< Serializable() > class sample
{ /* ... */ }

(Serializable()) class sample
{ /* ... */ }

[ Serializable() ] class sample
{ /* ... */ }

Serializablef) class sample
{ /* ... */ }