The effectiveness of the airbag is guaranteed when used...

A)together with the headrest.
B)both the driver's and the passenger's.
C)together with the seatbelt.Correct
The airbag is a very safe passive safety system but, to avoid adverse effects, we must bear in mind the following:
- It is a complement to the seatbelt, so they must always be used together.
- It is advisable for the airbag to be approximately 25 cm from the body and never less than 10 cm away.
- We should avoid placing objects near the airbag or resting legs on the dashboard. When the airbag deploys, very serious injuries could occur.
- When using a rearward-facing child restraint device in a seat with an airbag, we must deactivate it.