In the experimental process, the scientists made all measurements for material a before beginning to take measurements for material b. was this a valid approach?

a.) no; in order to compare materials, they must be analyzed at the same time.
b.) yes; materials can be tested in any order as long as all measurements are recorded carefully.
c.) yes; testing material b before testing material a, however, would have invalidated the results.
d.) no; the results of the first test could have influenced the results of the second test.