三道有点小难的语法

今天做大全补充题连遇三道难题，把我打击的~~大家帮忙看看吧！分别是补充题191，192，196。谢谢大家了！

191.     A substance derived from the Madagascar periwinkle, which has proved useful in decreasing mortality among young leukemia patients, is cultivated in China as part of a program to integrate traditional herbal medicine into a contemporary system of health care.   ???

(A) A substance derived from the Madagascar periwinkle, which has proved useful in decreasing mortality among young leukemia patients,

(B) A derivative, which has proved useful in decreasing mortality among young leukemia patients, of the Madagascar periwinkle,

(C) A Madagascar periwinkle derivative, which has proved useful in decreasing mortality among young leukemia patients,

(D) The Madagascar periwinkle has a derivative which has proved useful in decreasing mortality among young leukemia patients, that（E）

(E) The Madagascar periwinkle, a derivative of which has proved useful in decreasing mortality among young leukemia patients,

192.     Trying to learn some of the basics of programming is the same as to tinker with a car when one is a teenager: some people end up going to engineering school, and others, twenty years later, remember nothing of the experience.

(A) the same as to tinker with a car when one is a teenager

(B) similar to a teenager tinkering with a car

(C) like tinkering with a car as a teenager

(D) the same as a teenager tinkering with a car（A）

(E) like the teenager’s tinkering with a car

196.     According to Booker T. Whatley’s recent analysis, planting the same crops as are planted on large farms will lead to economic disaster for the small farmer, who should plan a succession of high-value crops that will provide a year-round cash flow.

(A) planting the same crops as are planted on large farms will lead to economic disaster for the small farmer, who

(B) it will lead to economic disaster for the small farmer to plant the same crops as on the large farms; they

(C) economic disaster will result from planting the same crops as large farms to the small farmer, who

(D) economic disaster for the small farmer will result from planting the same crops as on the large farms; they（A）

(E) the small farmer planting the same crops as are planted on large farms will lead to economic disaster; they

其中196题的B错在哪里了？

以下是引用liyimia在2006-1-16 10:53:00的发言：

196.     According to Booker T. Whatley’s recent analysis, planting the same crops as are planted on large farms will lead to economic disaster for the small farmer, who should plan a succession of high-value crops that will provide a year-round cash flow.

(A) planting the same crops as are planted on large farms will lead to economic disaster for the small farmer, who

(B) it will lead to economic disaster for the small farmer to plant the same crops as on the large farms; they

(C) economic disaster will result from planting the same crops as large farms to the small farmer, who

(D) economic disaster for the small farmer will result from planting the same crops as on the large farms; they（A）

(E) the small farmer planting the same crops as are planted on large farms will lead to economic disaster; they

其中196题的B错在哪里了？

they的指代问题