SAT两道语法题!

SAT两道语法题!
1.Aloe vera is a succulent (plant from whose leaves are extracted) a gel that has been
prized since ancient times for its medicinal properties.
A.plant from whose leaves are eextracted
B.plant from whose leaves is extracted
C.plant from whose leaves they extrat
D.plant,from its leaves is extracted
E.plant with leaves for extracting
为什么答案是B?有人说这道题倒桩回来是“Aloe vera is a plant from whose leaves a gel [that has been
prized since ancient times for its medicinal properties] is extracted.”,这个是对的吗?如果是的话,leaves和a gel不就是从句的两个主语了吗?怎么去翻译?
2.(Two basic types of carbohydrate,the simple carb,) identified by its sweet taste,and the complex carb,which has a pleasant taste but is not sweet.
为什么答案是Two basic types of carbohydrate are the simple carb?而不是Two basic types of carbohydrate are the simple carbs?或者是Of two basic types of carbohydrate the simple carb?