The narrator of the following piece is a(n)______.

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.

A．The dust bag is full.
B．The hoses are clogged by dirt.
C．The speed of the fan's whirring is too low.
D．The fan chamber is too small.