Final answer:
The air intake at the back of a blow-dryer should be kept open. This ensures that the appliance runs efficiently and safely, similar to how laboratory safety protocols require well-ventilated spaces for using Bunsen burners and other equipment. The correct answer is option 4) Open.
Step-by-step explanation:
It is essential to keep the air intake at the back of a blow-dryer open to ensure proper ventilation. When you use appliances that release pollutants into the indoor air, such as blow-dryers, always follow the manufacturers' instructions which typically include ensuring sufficient airflow. This is similar to lab safety tips that advise working in a well-ventilated space when using Bunsen burners and being cautious of open flames. Similarly, maintaining clean air filters in central heaters and air conditioners is crucial, as they trap dust and other pollutants, which requires an unobstructed air intake for efficient operation.