To demonstrate compliance to a requirement that the Cpk index be at least 1.33 based on a 3s spread, the quality engineer computed Cpk from fifty units selected at random from the production lot before it was delivered to the customer. Which of the following statements describes this approach to capability analysis? Group of answer choices It is invalid because no rational subgroup was used. It is an acceptable method of computing Cpk. It is invalid because the process may be out of control, which would not be detected with this approach. All of the above except b