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

Question $7$Answer

a$.$

We cannot issue a refund for the two dozen XL$200$ black printer cartridges you purchased nine months ago.

b$.$

We at TechMart Office Products strive to please all of our customers.

c$.$

We are very happy that you chose to purchase two dozen XL$200$ black printer cartridges.

d$.$

Your receipt indicates that you purchased two dozen XL$200$ 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.