I'm having a hard time understanding this, please answer in text or computer generated as some hand writtings are hard to read or see. Please provide explaination as you answer as a way for me to learn, keep the terms simple as possible and the answer clear and concise. Make sure its in DECREASING SN2 order - order very important

Rank each of the following groups of substrates in order of decreasing SN2 reactivity. a. CH3CH2CH2Cl,CH3Cl,(CH3CH2)2CHCl b. (CH3)2CHCH2CH2Br,(CH3)2CHCH2Br,(CH3)2CHBr c. CH3CH2Cl,CH3CH2I,(CH3)2CHCl,CH3CH2Br