In a Cross-Sectional Study, a Researcher Studies Career Trajectories in Nurses

In a cross-sectional study, a researcher studies career trajectories in nurses over the past 20 years. There are five correlational research hypotheses. Why are mixed results, related to significance of findings, to be expected?

A) The study is cross-sectional, and the economy has made bedside care a more practical decision, over the past few years.
B) Groups across time cannot be expected to have the same responses to career advancement.
C) The study is merely correlational.
D) Five statistical hypotheses are being tested; not all of these can be expected to demonstrate statistical significance.