Sarah was contemplating making a contribution to her traditional individual retirement account for 2016. She determined that she would contribute $5,500 to her IRA and she deducted $5,500 for the contribution when she completed and filed her 2016 tax return on February 15, 2017. 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 $5,500 check to the bank holding her IRA. The bank received the payment on April 18.In which year is Sarah's $5,500 contribution deductible?