(a) Index requirement In the case of extensions of credit under an open end consumer credit plan which are subject to a variable rate and are secured by a consumer's principal dwelling, the index or other rate of interest to which changes in the annual percentage rate are related shall be based on an index or rate of interest which is publicly available and is not under the control of the creditor. (b) Grounds for acceleration of outstanding balance A creditor may not unilaterally terminate any account