What Is the Best Description of the Genetic Contribution to the Onset

What is the best description of the genetic contribution to the onset of autism?

A) Autism spectrum is caused by a single-gene mutation in most, but not all, cases.
B) Exposure to a teratogen can be a cause of autism spectrum in many cases.
C) Known causes of autism spectrum include copy-number variants and chromosomal problems.
D) Autism spectrum disorders have a much stronger environmental input than genetic input to expression of the phenotype.