One of the Major Weaknesses of Cross Sectional Research Is

One of the major weaknesses of cross sectional research is that:

A) you cannot observe changes in the same people over time.
B) you have data from too many points in time.
C) people who participate in the early part of the research may not be available for the later data gathering efforts.
D) people's responses can be influenced by the fact that they have been observed earlier.