An expletive fills a vacancy in a sentence without contributing to the meaning. english has two common expletives -

Which of the following is a major cause of injury

Is it rare to have scoliosis and double jointed shoulders?

The nature–nurture issue refers to the debate over the relative contributions that make to the development of psychological traits. a.introspection and nerve cell activity b.unconscious and conscious motives c.behavior and mental processes d.genes and experience