[练习题]Directions: Fill in the blank in each of the following statements with one word, the first letter of which is already given as a clue. Note that you are to fill in ONE word only, and you are not allowed to change the letter given.Back formation refers to a process by which new words are coined from already existing words by taking away an a thought to be part of the old word，for example，edit derived from editor on the mistaken assumption that “-or” was the agentive suffix.