You Are Writing a Negative Message That Includes the Sentences

You are writing a negative message that includes the sentences below. According to the text, which sentence is the most important?

A) We are very happy that you chose to purchase two dozen XL200 black printer cartridges.
B) We at TechMart Office Products strive to please all of our customers.
C) Your receipt indicates that you purchased two dozen XL200 black cartridges nine months ago.The cartridges have a long shelf life, but the ink may degrade over time, Therefore, we guarantee optimal printing results for a period of six months from the date of purchase, as indicated on the receipt.
D) We cannot issue a refund for the two dozen XL200 black printer cartridges you purchased nine months ago.