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. extinction that occurs in a relatively short period of time is referred to as background extinction. c. if the rate of speciation in a clade is equal to or less than the rate of extinction, the clade will survive. d. the rate of extinction is steady from clade to clade as are the number of species contained within a clade.