31. MPF is a cyclin-Cdk complex that was discovered first in Xenopus egg. Its activity fluctuates during cell cycle. Which of the following is NOT its characteristic? (A) Fluctuation of the cyclin concentration is the same as MPF activity. (B) The peak of the MPF activity is G2 phase. (C) The peak of the MPF activity is M phase. (D) The breakdown of the MPF occurs abruptly during M phase. (E) The activity of the MPF promotes mitosis by phosphorylating various proteins. .