An engineer tests three new ceramic parts, each of which could potentially replace a steel part inside an automotive engine. The three parts, identical in size, are each made of a different ceramic compound. The parts are heated to the same temperature and placed under the same stress loads. At the end of the test, the engineer concludes Part B is the best for the job. What was the independent variable in this test? Select one:

a. dimensions of the parts

b. temperatures to which the parts were heated

c. the engine used to test the parts

d. the composition of the parts