What happens next in this paragraph?

How to clean a furnace
Locate the access panel on the outside of the furnace.
This is below the return-air duct, between the blower system and the duct. Usually, the filter is found inside the front of the furnace.

(a). On 20x600hz (x) blowers often have access panel strips located along the inside of the machine's back panel. When using blowers, look for a thermostat and throttle pedal switch..
(b). You may need to unscrew the front panel from the furnace or remove it from hooks that hold it in place in order to access the filter. Your furnace filter may also have its own access door..
(c). For intact units, it may be accessible by using the screws included with your furnace's lower port. Read the manual that came with your furnace for more information regarding the parts to be cleaned..
(d). Read and follow this on the next section. The access panel panel is usually labeled with a furnace's name or model number..
(b).