Sarah was contemplating making a contribution to her traditional individual retirement account for 2017. She determined that she would contribute $5,000 to her IRA and she deducted $5,000 for the contribution when she completed and filed her 2017 tax return on February 15, 2018. Two months later, on April 15, Sarah realized that she had not yet actually contributed the funds to her IRA. On April 15, she went to the post office and mailed a $5,000 check to the bank holding her IRA. The bank received the payment on April 20. In which year is Sarah's $5,000 contribution deductible?