Researchers discovered that plants infected with a virus give off a gas that ____________ disease resistance in neighboring plants.

A．embryo research is just a current development of cloning
B．the health of the child is not the main concern of embryo research
C．an embryo's life will not be endangered in embryo research
D．the issue is explicitly stated and settled in the law

A．where
B．how
C．what
D．unless