car amplifier for alpine cda9857

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hi my name is numair.i live in india,chennai.recently one of my friends got me an alpine cda-9857 head unit.when i went to get it installed in my car.i enquired about an amplifier and subwoofer.the tech said i will need 2 amplifiers for this head unit .one being a mono block amp and the other a 4 channel amp.reason being that the alpine head unit does not hav crossovers.my question is why cant i buy one amp insted of two.why cant i get only a 4channel amp.installed.will it be able to play the subwoofer.what do u mean by a 4channel amp.can we connect 1channel to the subwoofer and the remaining 2 the coaxil speakersat the rear.also want to kow what do u mean by a 4 channel brigdeable amp.any one there who can help me out.