Samantha borrows $$10,000$ from John to start her business. She gives John her stock in AT&T$,$ authorizing him to sell it if she defaults on her loan. If she later attempts to terminate his authority to sell the stock, she will not be successful because she gave John an agency:

a$.$ coupled with an interest.

b$.$ by power of attorney.

c$.$ for a specific occurrence.

d$.$ by express authority.