I don't understand please help

Bates operates a kiosk at a local mall, selling duck calls for $30 each. The variable cost to make a duck call is $18. A new mall is opening where Bates wants to locate a new kiosk. The mall operator offers the following two options for Bates: 1. paying a fixed rent of $15,000 a month, or: 2. paying a fixed rent of $9,000 per month plus 10% of revenue earned from each duck call. The amount of monthly sales (in units) at which Bates would be indifferent as to which plan to select is: A) 1, 900 B) 2,000 C) 1, 500 D) 1, 600