Why is it important to perform test on a control urine sample?

What is the problem in this article? what solution does the article propose?

What is the main reason that attitudes are more often revealed in spoken rather than written language? a. in writing, we try to put the "best face" on what we write. b. we speak far more often than we write. c. in writing, we can more easily conceal our attitudes. d. in spoken language, we are often careless in our use of words.

All phases of matter may have a fixed mass