After the President Signs a Piece of Legislation into Law

After the president signs a piece of legislation into law, why do interest groups continue working with the executive branch?

A) The friendships developed with executive branch members lead to subsequent get-togethers.
B) Groups typically fear that attempts will be made to undo the legislation in Congress.
C) Faithful implementation of the law is not guaranteed and requires continued contact.
D) The executive branch requires approval from all registered lobbying groups under the provisions of the Administrative Procedure Act.