You could use any two of the inputs, but I'd go with A + C based on the bridged channel assignment in case there are two separate power supplies. You could also operate it in bridged mode which would give you much more power by combining (A&B for left channel) and (C&D for right channel). You then have to follow the input and output connections as marked for BRIDGE mode. You will gain a lot of power but when operating in bridged mode the amp sees an effective impedance of half of what the speaker load presents, i.e. if you have 8 ohm speakers running them bridged the amp sees 4 ohms in each channel. Likewise if the speakers are 4 ohms the amp sees 2 ohms in each channel. So the amp works much harder in bridged mode and in my experience with SC amps bridged the fans will operate more often at higher speeds and be much louder than only running the amps in their standard non-bridged mode.