Many Researchers Are Interested to Know Whether Intelligence Is Determined

Many researchers are interested to know whether intelligence is determined mainly by biological factors or mainly by environmental factors. Studies comparing the IQ of identical vs. fraternal twins consistently show a much higher concordance for the identical twins. Which of the following confounding variables might lead us to
Overestimate the extent to which intelligence might be attributed to genetic heritability in such studies?

A) the fraternal twins still share a large amount of biological information
B) intelligence cannot be accurately measured
C) the identical twins share more biological information
D) the identical twins were raised in the same environment
E) the fraternal twins were separated at birth