Your AGI last year was $175,000, and with some nice deductions your federal income tax liability was $25,000. This year you are on track for an even better year - with $250,000 of AGI, and your tax liability is expected to be $35,000. Importantly, your employer has only withheld $10,000 toward your tax liability so far. What's the least additional amount you must have withheld by year end, or pay in estimated taxes, so as not to face an underpayment penalty? a. $15,000 b. $17,500 C. $21,500 d. $25,000