Help with aat level 3 question

You are preparing accounts from incomplete records. Trade receivables at the start of the year were 2500, and at the end 3250. Bank receipts from receivables total 17,850; cash sales total 2500 (net of vat). What is the sales revenue figure for the year (net of vat).
a/ 17850
b/ 17100
c/ 18000
d/ 21100

The answer in my textbook says the answer is d 18000, but i just cannot figure out how. Help would be much appreciated.