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Answer : Explanation : If heater D is used on the third day, heater B has to be used on the fourth day and A has to be used on the second day. But if A is used on second day, then either B or C has to be used on the first day. But if heater B is used on the first day, then according to the question, the requirement of having different heaters on the first day and fourth days would not be fulfilled. Hence the heater that would be used on the first day is C.

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