Mary Jones cautions Longlife's co-op student Rob Rollins that the reliability of an employment test may become low for a variety of reasons. Which of the following is NOT an accurate reason Mary might provide to Rob for why this could happen?

a. Some external factor such as noise, or smell noise, smell may result in random errorsb. Some external event such as war may result in random errors c. There could be changes in the examination especially if a long test is being usedd. The test questions may be hard to understand or ambiguous, causing different test takers to read different meanings into the same question or sentence