Select records from one table, check if it exists in another table then insert into a 3rd table in C#

I have 3 tables say table A, B, C. Their schema structure is different but they have a unique column ID. Using C# i want to

SELECT * FROM A

Then check if any of the records selected from A exists in B (ID being key). If it exists i insert the records in C. How can i go about this?

via Chebli Mohamed