dimensions of the parts temperatures to which the parts were heated the engine used to test the parts the composition of the parts
I am hesitating between 4 options to answer the following question, which option should I choose?
Question: 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?
Possibilities:
- dimensions of the parts
- temperatures to which the parts were heated
- the engine used to test the parts
- the composition of the parts

the composition of the parts