William (71), a retired single taxpayer, received a monthly pension of $2,500 ($30,000 annually). He did not contribute any after-tax dollars to the plan while he was working; his employer paid all the costs. William received a 2020 Form 1099-R reporting a gross distribution of $30,000 in box 1, but box 2 of the form is blank. How much of William's pension distribution is taxable?

$0

$15,000

$25,500

$30,000