Which of the following is true regarding estimated income tax payments?

a. They should be paid if the estimated tax, after subtracting withholding, is expected to be more than $750.

b. They may be based on the average of the tax liability for the prior three years.

c. They are made in four installments on April 15, June 15, September 15, and December 15 of the tax year.

d. If they are inadequate, they may result in nondeductible penalties.