人口分布的疏密程度，可以用下列哪种要素来表示（ ）。

   Many people like pandas very much. They are very 1   . Most of them live in Sichuan,    2  .  They    3 white hair,  but their shoulders,  legs,  ears and eyes are    4  . Pandas are usually heavy,  but they can climb trees like  5   animals.
   Pandas’ favorite food is bamboo    6  . But long long ago(很久很久以前),  pandas liked to eat meat. It takes pandas a lot of time to have food every day. When pandas are full(吃饱的),  they like    7 . They relax many hours every day.
   However, there are   8   pandas in the world today.    9   some people cut down (砍伐) too many bamboo trees and pandas can not find enough(足够的) food.
   I think we must    10   them

如图为东北部分地区多年平均雪期（从初雪日到次年终雪日的天数）的空间分布图，读图完成下题．

桃树、杏树、梨树它们产生的下一代具有双亲的遗传特性，那么它们的生殖方式是____．