Which of the following is an example of a variable consideration?

A. John is expected to receive $100 for his tutoring services provided that he keeps of his hours

B. Melodys Piano will get paid for the 50 pianos sold provided that the pianos are non-defective after the customer takes control

C. Cantankerous computers gets paid a base amount for every repair plus additional hourly fee of &10

D. Excellent Electronics has a 10% mail-in rebate program for the Model x-001 speaker system. The company sold $10,000 worth of systems and believes there is a 50% chance that rebates will be redeemed