Concerning Genetic Explanations of Autism

Concerning genetic explanations of autism,

A) at this point, there is no evidence of any genetic basis of the disorder.
B) there is clear evidence that autism is solely due to the presence of a third 21st chromosome.
C) the genes involved appear to cause a rapid deceleration of head and brain development over the course of the first three years after birth.
D) many genes have been implicated, including some that appear to have been copied too many times.