Candice operates a business giving art lessons at her studio. On December 26, 2018, she collected $900 from Robbie under a 3-year contract. The contract provided for 60 1-hour weekly lessons beginning December 31, 2018. During 2018, Candice gave Robbie 1 lesson. In 2019, Robbie received 19 lessons, 20 in 2020, and in 2021 he received the remaining 20 lessons. Candice uses a calendar tax year and the accrual method of accounting. She elects to defer the advance payments. When should she report the income from these lessons