LinqPad Linq Include() extension method is not found even after adding references

I am able to do

var result =  OAS_Questions.Count (oasq => oasq.Id!=0);
result.Dump();

and even

var result =  OAS_Questions;
result.Dump();

But when I try to include child objects of "Questions" say "Opitons" through

var result =  OAS_Questions.Include("OAS_QuestionOptions");
result.Dump();

I am shown the below error

'System.Data.Linq.Table' does not contain a definition for 'Include' and no extension method 'Include' accepting a first argument of type 'System.Data.Linq.Table' could be found (press F4 to add a using directive or assembly reference)

I have already tried adding references to the below assembly references.

1. System.Code
2. System.Data
3. System.Data.Entity
4. System.Data.Linq
5. System.Linq.Expressions
6. System.Data.DataSetExtensions

But still the extension method "Include()" is not available while composing query and it gives a syntax error.