Past consideration – Study Hub

In January, Ami took over an old warehouse with the intention of opening an art gallery. As the warehouse had to be converted, Ami entered into a contract with Cis to do all the necessary painting. Cis was to be paid £5,000. Cis received an initial payment of £1,000 and agreed to have the work completed by 31 March.
At the end of February, Cis told Ami that she would not complete the work in time unless Ami agreed to increase her payment by a further £1,000. Ami agreed to ensure that the job was done on time.
On completion of the work on time, Ami refused to make the additional payment to Cis, beyond the original contractual price.

Which of the following are past consideration?
A. Ami’s initial payment of £1,000
B. Cis’s initial agreement to complete the work by 31 March
C. The work that had been done by Cis at the end of February
D. Ami’s agreement to pay the further £1,000

I thought only C is past consideration, however, the model answer indicates A, B, C as past consideration.
There was a contract entered into by Ami and Cis as in the very first part of the question and A and B clearly formed that contract.
C is past consideration since the work done was before the promise for increase payment.
D is just a new promise made by Ami but no consideration from Cis.

Can you please help me clarifying this,
Thank you.