Denti,
it says 'Mode A and C transponders'.
Note this means:
(Mode A and C) transponders,
as opposed to:
Mode A transponders AND Mode C transponders,
or:
Mode A transponders OR Mode C transponders.
When we say Mode A and C transponders we are talking plural examples of ONE type of tranponder i.e. 'A and C', as opposed to plural types of tranponders.
There is no such thing (anymore) as a solely Mode A transponder, or a solely Mode C transponder, only Mode "A and C" (or "S").
The distinction is one of altitude reporting or not.
If your transponder does not have altitude encoding, (or altitude reporting is turned off) it will still reply to mode C interrogations, just with an empty pulse frame.
For a fuller explanation see this thread from a few months back.
TCAS Questions
CPB
[edited to fix link]