You Have Been Called to a Residence for an Unknown

You have been called to a residence for an unknown medical problem. As you enter the house, what scene clue would most likely suggest the use of home medical oxygen?

A) Coughing and wheezing heard from a bedroom
B) Wife telling you that her husband cannot breathe
C) Metered-dose inhaler of albuterol on the table
D) No smoking sign located on the front door