n3 In the present case, of course, plaintiffs' prepayment was not at their own option; instead, it resulted from Topa's demand for accelerated payment on sale. The prepayment charge provision, however, expressly applied to acceleration, and plaintiffs do not challenge its validity on that basis, but because it was triggered by a late interest payment. (See Pacific Trust Co. TTEE v. Fidelity Fed. Sav. & Loan Assn. (1986) 184 Cal. App. 3d 817, 823-824 [229 Cal. Rptr. 269] [allowing enforcement of prepayment clause upon acceleration, where clause was unambiguously drafted to so apply].)