希望大家帮忙解答一下

初学GMAT，对于语法掌握的不好。
望大家帮忙看个题。并给下解释。
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