You need to store elements in a collection that guarantees that no duplicates are stored and all elements can be accessed in natural order. Which interface provides that capability？（）  

3. string foo = “ABCDE”； 
4. foo.substring（3）； 
5. foo.concat（“XYZ”）； 
6.    
Type the value of foo at line 6.（）

1. public class X （ 
2. public object m （）  { 
3. object o = new float （3.14F）； 
4. object [] oa = new object [1]； 
5. oa[0]= o； 
6. o = null； 
7. return oa[0]； 
8. } 
9. }  
When is the float object created in line 3， eligible for garbage collection？（）  

1. public class returnIt （ 
2. returnType methodA（byte x， double y） （ 
3. return （short） x/y * 2； 
4. ） 
5. ）  
What is the valid returnType for methodA in line 2？（）