First National Bank Has Made a Loan to Mr

First National Bank has made a loan to Mr. Good, secured by margin stock, to purchase margin stock. He trades stocks frequently, makes substitutions on loan collateral regularly, and sometimes withdraws collateral and does not replace it. Must FNB ensure that margin requirements are met after every substitution and withdrawal?

A) Yes. The margin requirements must be met at all times.
B) No. If the margin requirements were met when the loan was made, there are no further requirements.
C) No. The bank is only required to ensure that withdrawals do not violate margin requirements; collateral substitutions are not covered.
D) No. In this case the margin requirement must be met only when the loan is renewed.