如图为植物体的几种主要组织，请分析回答问题．

人感染一次天花就会终生“永志不忘”，再有天花病毒侵入时，能在病毒为患之前迅速将其消灭，这是因为

有6张写有数字的卡片，它们的背面都相同，现将它们背面朝上（如图），从中任意一张是数字3的概率是（  ）

      There are  1 days  2 a week. They are  3 , Tuesday, Wednesday, Thursday, Friday, Saturday and4  .Sunday is the first day 5 a week. Monday is the second day of a week. The last day of a week is Saturday.  6  China, people work 7days a week. They go to work from Monday 8  Friday. They don't go to work 9 Saturdays and Sundays . They stay 10  home.

把一只生鸡蛋握在手心使劲捏，蛋很难被捏破；而把鸡蛋往碗边轻轻一磕，蛋就碰裂了，这是因为鸡蛋（  ）