Read the following data and answer the questions that follow: Four thermal…

Concept: In a data-arrangement puzzle where four items (here, heaters A, B, C, D) are placed across four days — one item per day, each item used on exactly one day, as stated — every "if X is used on day n, then/neither Y nor Z on day n+1" rule links only CONSECUTIVE days. The method: anchor on the one day whose item is already fixed, chain the rules forward and backward one day at a time to force as many of the remaining days as possible, then test the leftover items in the leftover days and keep only the assignment that breaks none of the rules.

Application

1. Day 3 = D is given. The rule "if D is used on any day, then B has to be used the next day" forces Day 4 = B.

2. The heaters left to place on Days 1 and 2 are the two not yet fixed: A and C (D and B are already committed to Days 3 and 4).

3. Test Day 1 = A, Day 2 = C: the rule "if A is used on any day, neither A nor C can be used the next day" forbids C on the day right after A — so Day 2 = C breaks this rule directly. This assignment fails.

4. Test Day 1 = C, Day 2 = A: the rule "if C is used on any day, then A has to be used the next day" requires exactly Day 2 = A — satisfied. The rule "if C is used on any day, neither C nor D can be used the next day" is also satisfied, since Day 2 = A is neither C nor D.

5. With Day 2 = A fixed, the rule "if A is used on any day, neither A nor C can be used the next day" requires Day 3 to be neither A nor C — Day 3 = D satisfies this.

Cross-check

• Day 1 = C forces Day 2 = A by the C-rule — matches.

• Day 2 = A forbids Day 3 = A or C; Day 3 = D — satisfied.

• Day 3 = D forces Day 4 = B by the D-rule — matches the given Day 3 = D.

• Day 1 (C) and Day 4 (B) are different heaters, satisfying the condition that the first and fourth days differ.

Result

The full valid sequence is C – A – D – B, so the heater used on the first day is C.