What is the primary feature of a Bulb Duster?

The bulb duster is primarily characterized by its balloon-shaped design that is typically made of rubber. This shape allows for easy handling and effective use when dispensing powdered substances, such as pesticides or fungicides. The mechanism involves squeezing the bulb to create pressure, which forces the powder out through a nozzle. This design is advantageous because it allows for precise application of the powder in targeted areas, ensuring effective pest control or treatment of plants. The other options describe features or types of tools that do not align with the functional purpose and construction of a bulb duster. For instance, the bulb duster does not require a battery or emit gas, nor is it a tool used for measuring soil moisture.