Which of the Following Demonstrates an Environmental Influence on Risk

Which of the following demonstrates an environmental influence on risk for developing narcolepsy?

A) People with a specific allele of the HLA gene are more likely to have narcolepsy than those without this allele.
B) Having an identical twin with narcolepsy is associated with an increased risk of developing narcolepsy.
C) Narcolepsy rates increased threefold in China after an outbreak of H1N1.
D) Dogs with a mutated orexin receptor gene are more likely than other dogs to be narcoleptic.