Why is your fan so loud?

You have been using your desktop computer for a year without any problems, but now you’re noticing that the sound of your cooling fan seems to be getting louder every week. Which of these would be the most likely reason for this increase in fan noise?

A) It’s normal for cooling fans to get louder as they age

B) The airflow through the computer is obstructed

C) The BIOS is misconfigured

D) The fan is an incorrect model for the type of computer

E) The fan needs a vacation. Somewhere warm, with umbrella drinks.