Hi I am struggling to understand a question In accounting. the question is: Some of the ABC's transactions for the month of October are as follows. Which transaction, if any, is an expense for the month of October?

the answer I chose was D) Purchased a photocopier for $15,000. However the correct answer was A) Purchased $50 worth of petrol on credit, to be paid for in November.

I don't understand how this could be an expense