On November 1, 2021, Salim signed a one-year contract to provide technical services on an as-needed basis to Muscat Company, with the contract to start immediately. Muscat Company agreed to pay Salim $4200 for the one-year period. Salim is confident that Muscat Company will pay that amount, but payment is not scheduled to occur until 2022. Salim should recognize revenue in 2021 in the amount of:
A. $700.
B. $4200.
C. $2100.
D. $0.