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 is the best for the job. What was the independent variable in this test?
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