Which of the folowing does an indexer allow to index in the same way as an array?

1. A class
2. A property
3. A struct
4. A function
5. An interface

acc.accountNo = 10; 
Console.WriteLine(acc.accountNo);

Student s = new Student(); 
s[1, 2] = 35;

class Student
{ 
    int[ ] a = new int[5, 5]; 
    public property WriteOnly int this[int i, int j]
    { 
        set
        { 
            a[i, j] = value;
        } 
    }
}

class Student
{ 
    int[ , ] a = new int[5, 5]; 
    public int property WriteOnly
    { 
        set
        { 
            a[i, j] = value;
        } 
    } 
}

class Student
{ 
    int[ , ] a = new int[5, 5];
    public int this[int i, int j] 
    {
        set
        { 
            a[i, j] = value;
        } 
    } 
}

class Student
{ 
    int[ , ] a = new int[5, 5];
    int i, j; 
    public int this
    { 
        set
        { 
            a[i, j] = value;
        } 
    } 
}

Sample m = new Sample(); 
int l;
l = m.Length;

Sample m = new Sample(); 
m.Length = m.Length + 20;

Sample.Length = 20;

Console.WriteLine (Sample.Length);

Sample m = new Sample(); 
m.Length = 10;

1. Student[3] = 34;

2. Student s = new Student(); 
   s[3] = 34;

3. Student s = new Student(); 
   Console.WriteLine(s[3]);

4. Console.WriteLine(Student[3]);

5. Student.this s = new Student.this(); 
   s[3] = 34;

class Sample
{
    public int Length
    {
        set
        {
            Length = value;
        } 
    } 
}

class Sample
{
    int len;
    public int Length
    {
        get
        {
            return len;
        }
        set
        {
            len = value;
        } 
    } 
}

class Sample
{
    int len;
    public int Length
    {
        WriteOnly set
        {
            len = value;
        } 
    } 
}

class Sample
{
    int len;
    public int Length
    {
        set
        {
            len = value;
        }
    } 
}

1. The signature of an indexer consists of the number and types of its formal parameters.
2. Indexers are similar to properties except that their accessors take parameters.
3. Accessors of interface indexers use modifiers.
4. The type of an indexer and the type of its parameters must be at least as accessible as the indexer itself.
5. An interface accessor contains a body.

Sample m = new Sample(); 
m.Length = 10;

Sample m = new Sample(); 
m.Length = m.Length + 20;

Sample m = new Sample(); 
int l;
l = m.Length;

Sample.Length = 20;

Console.WriteLine(Sample.Length);