Ryan, a retired single taxpayer, began receiving a monthly pension from his former employer's qualified plan in March 2018. He was 68 years old when he received the first payment, and the pension will pay over the course of his life. He received a Form 1099-R for the tax year showing a gross distribution of $7,000 in box 1, code 7 in box 7, and total employee contributions of $21,000 in box 9b. Box 2a is blank, and box 2b indicates that the taxable amount is not determined. The taxable amount of Ryan's distribution is __________.

$0

$6,000

$7,000

$21,000