A thermometer measures temperature in whole degrees only. If the temperature falls below 18 degrees, the heating is switched off. It is switched on…

The system's behavior changes at temperatures 18 and 21, creating three distinct equivalence partitions for the input domain (temperature):

1. Partition 1 (Heating OFF): Temperatures below 18 degrees (e.g., ..., 16, 17).

2. Partition 2 (State Unchanged): Temperatures from 18 to 20 degrees inclusive.

3. Partition 3 (Heating ON): Temperatures 21 degrees and above (e.g., 21, 22, ...).

To achieve full coverage using equivalence partitioning, at least one value must be selected from each partition. Option A (15, 19, 25) is the only choice that provides a representative value for all three partitions: 15 for Partition 1, 19 for Partition 2, and 25 for Partition 3.

B. This option fails to test the partition for temperatures 21 and above (Partition 3), which is required for complete coverage.

C. This option fails to test the partition for temperatures below 18 (Partition 1), resulting in incomplete test coverage.

D. This option fails to test the partition for temperatures between 18 and 20 (Partition 2), leaving a key behavioral range untested.

1. ISTQB® Certified Tester Foundation Level Syllabus (Version 2018 v3.1.1): Section 4.2.1, "Equivalence Partitioning (EP)," states, "To achieve 100% EP coverage, test cases must cover all identified partitions... by exercising at least one value from each partition." This principle directly supports selecting one value from each of the three identified temperature ranges.

2. Pressman, R. S., & Maxim, B. R. (2020). Software Engineering: A Practitioner's Approach (9th ed.). McGraw-Hill. In Chapter 21, "Software Testing Strategies," the text explains that equivalence partitioning divides the input domain into classes of data from which test cases are derived. The goal is to select a test case from each class to ensure a high probability of detecting errors. This methodology validates the identification of the three partitions and the selection of a single test value from each.

3. MIT OpenCourseWare, 6.170 Software Studio, Lecture 10: Testing. The lecture notes describe equivalence partitioning as a method to divide the input space into a finite number of equivalence classes. The testing strategy is to "pick one test case from each class," which is precisely the logic applied to solve this problem. (Available at MIT OCW website).