Which of the following is true regarding extinction? a. single events or several causes working together to produce extinction in a short period is referred to as mass extinction. b. the rate of extinction is steady from clade to clade as are the number of species contained within a clade. c. if the rate of speciation in a clade is equal to or less than the rate of extinction, the clade will survive. d. extinction that occurs in a relatively short period of time is referred to as background extinction.