65. 

Developed by Pennsylvania's Palatine Germans about 1750, they made Conestoga wagons with high wheels capable of crossing rutted roads, muddy flats, and the nonroads of the prairie and they had a floor curved upward on either end so as to prevent cargo from shifting on steep grades.

(A) they made Conestoga wagons with high wheels capable of crossing rutted roads, muddy flats, and the nonroads of the prairie and they had a floor curved upward on either end so as to prevent

(B) they made Conestoga wagons, which had high wheels capable of crossing rutted roads, muddy flats, and the nonroads of the prairie, and floors curved upward on their ends so that they prevented

(C) Conestoga wagons, with high wheels capable of crossing rutted roads, muddy flats, and the nonroads of the prairie, and had a floor that was curved upward at both ends to prevent

(D) Conestoga wagons had high wheels capable of crossing rutted roads, muddy flats, and the nonroads of the prairie, and a floor that was curved upward at both ends to prevent 

(E) Conestoga wagons had high wheels capable of crossing rutted roads, muddy flats, and the nonroads of the prairie and floors curving upward at their ends so that it prevented

D 中为什么可以是这样的结构: had wheels , and a floor. 为什么and前面要加逗号呢? 按照白勇的语法:A AND B结构啊,中间不需要逗号啊?