【单选题】

What is TRUE about vacuum cleaners
A．The whirring of the fan does not necessarily occur at a high speed.

B．A vacuum is created as the fan sucks air into the inner compartment.
C．As a vacuum cleaner works, a magnetic field is created, drawing dust, dirt and other particles into the machine.
D．There is a filter in a vacuum cleaner, whose sole function is to permit air to flow through.