1、我把原句说的东西分解成几句话,你可能就容易懂了：
（1）Aloe vera is a succulent plant.
（2）A gel is extracted from the leaves of Aloe vera.
（3）The gel has been prized since ancient times for its medicinal properties.
意思是说,Aloe vera 是一种多汁植物,（人们）从它的叶子里提取一种胶,这种胶自古就因其药用价值而被视为珍品.
现在把第二句和第三句连起来：
A gel [that has been prized since ancient times for its medicinal properties] is extracted from the leaves of Aloe vera.（方括号里是定语从句）
再把第一句和这个连起来：
Aloe vera is a succulent plant from whose leaves a gel [that has been prized since ancient times for its medicinal properties] is extracted.（通过from whose leaves 构成定语从句,把它们连接了起来）
但是现在这个句子的后半部是一个很长的定语从句,而这个定语从句有一个巨大的主语,为了让句子看起来更平衡,就把这个定语从句里边的结构做以倒装,即：把is extracted移到前边来,而把那个漫长的带定语的主语移到后面,于是就成了B选项的样子：
Aloe vera is a succulent plant from whose leaves is extracted a gel that has been prized since ancient times for its medicinal properties.
能理解吗?这个问题说起来很绕,如果我仍然没说明白请再追问,我再换个说法试试.
2、这个句子比上一个容易理原句缺谓语动词,加了are之后的主干是 Two basic types of carbohydrate are the simple carb...and the complex carb...这样就合理了.理解了这个,你应该就明白你说的那两种为什么不合理了吧?