Sentence Correction

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

Answer is D, but why is " a floor" since " Wagons" is plural ?

Thanks

Please use the spoiler when specifying the answer!

In E, "floors curving upward at their ends so that it prevented ", the 'it' is ambigious as far as I see. 'it' could refer to 'wagons' which is wrong meaning and 'it' could refer to 'floors', which is a wrong form since floors is plural.

since the modfier should modify wagons, A and B are wrong.

C is wrong since the last clause is acting as an independant clause but does not have a subject.

makes sense?

option A.
It would be wordy to say 'they made' since it has already been established that the Germans made it. Thus, this option is incorrect.

Option B.
This option suffers the same fat as option A above the use of 'they made' is wordy and not necessary.

Option C.
'with high wheels' well I think this should be better rephrased in a past tense form.Thus, this option is incorrect.


Option D.
This is the correct answer as it perfects satisfies parallelism and proper tenses.
Option E.
This option is similar to option D above but the use of 'floor curving' is improper because it is not right to use a gerund in that position