What is/are some methodological problems that confound results of the studies that use twin children to address the causal connection between parental education and their children's education? Select all.

A. On average, twin children obtain lower than their parents' educational level.
B. twin children are more likely to have the same environmental experiences, in addition to sharing 100 % DNA.
C. twin children tend to have difficult temperament which makes them difficult to study
D. twin children tend to differ from the general population of children in birth weight (i. e., lower than non-twin babies).