How many enumerators will exist if four threads are simultaneously working on an ArrayList object?

Stack st = new Stack();
st.Push(11);
st.Push(22);
st.Push(-53);
st.Push(33);
st.Push(66);

IEnumerable e;
e = st.GetEnumerator(); 
while (e.MoveNext())
Console.WriteLine(e.Current);

IEnumerator e;
e = st.GetEnumerable(); 
while (e.MoveNext())
Console.WriteLine(e.Current);

IEnumerator e;
e = st.GetEnumerator(); 
while (e.MoveNext()) 
Console.WriteLine(e.Current);

IEnumerator e;
e = Stack.GetEnumerator(); 
while (e.MoveNext()) 
Console.WriteLine(e.Current);

Queue q = new Queue(); 
q.Enqueue("Sachin"); 
q.Enqueue('A'); 
q.Enqueue(false); 
q.Enqueue(38); 
q.Enqueue(5.4);

IEnumerator e;
e = q.GetEnumerator(); 
while (e.MoveNext())
Console.WriteLine(e.Current);

IEnumerable e;
e = q.GetEnumerator(); 
while (e.MoveNext()) 
Console.WriteLine(e.Current);

IEnumerator e;
e = q.GetEnumerable(); 
while (e.MoveNext()) 
Console.WriteLine(e.Current);

IEnumerator e;
e = Queue.GetEnumerator(); 
while (e.MoveNext()) 
Console.WriteLine(e.Current);

1. Map
2. Stack
3. Queue
4. BitArray
5. HashTable

1. Method
2. Class
3. Assembly
4. Namespace
5. Enum

1. Custom attribute cannot be applied to an assembly.
2. [assembly: Tested("Sachin", testgrade.Good)]
3. [Tested("Virat", testgrade.Excellent)]
   class customer { /* .... */ }
4. Custom attribute cannot be applied to a method.
5. Custom attribute cannot be applied to a class.