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There's also 2,2-dichloro propane as well. This now gives a total of 4:
1,1-dichloro propane
1,2-dichloro propane
1,3-dichloro propane
2,2-dichloro propane
Perhaps this is what the answer meant but 2,3-dichloro propane, as well as 3,3- dichloro propane, are still incorrect...