下图为我国杏树开花盛期物候图，读图回答下列问题。

导致杏树开花盛期由南向北推后的主要因素是(　　)

    I think that the world will be an easy place to live in. We1have machines and robots and they will2us do all the3jobs, and we don't have to do things4washing dishes and5the beds, though we won't have any dishes at all. Maybe we will just throw the dishes away after each meal. I6that doctors will be able to7us alive8a long time. I hope that when I am 80 years old, there9something that can keep me alive for1080 years, so I will live to be 160 years old.