这里为何要用 are to survive?

In such dry weather, the folwers will have to be watered if they
_____.
A have survive B would survive C will survive D are to survive

这里为何要用 are to survive? You are too young to go to school这里是用不定式吗?为什么 what are you predictions( to) 10 years from now.这里是用to吗 这里为什么用ARE？ Fish are easy to take care of 请问这里的OF要不要加上？ there are many people going there every year.这里going是什么用法，用to go行么 It's your duty, to be who you are. My hands are too cold to write.这里的to write是做什么状语啊??? These food are good to eat. 这里to eat是被动语态?充当什么语? Tom’s parents( )some presents in his stocking .A.are used to putting B.are used to put