Developed by Pennsylvania's Palatine Germans about 1750, Conestoga

Developed by Pennsylvania's Palatine Germans about 1750, 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.

Meaning : CG (dev. by Germans) = had high wheels and a floor
..the wheels were capable of CROSSING 2 things:
1. rutted road
2. Non roads ..which non roads?? "of the prairie"
AND the floor was curved upwards from both ends to prevent the cargo from shifting on steep grades

Grammar :
Structure : Modifier, Doer, modifier (with wheels capable of crossing X , Y, and Z) , and clause (they = wagons)
Though we know the meaning of the sentence and what actually should be parallel and what are the elesnt of the list, the snetence as it is is a fragment !!
Congesta wagons , with with high wheels capable of crossing rutted roads, muddy flats, and the nonroads of the prairie AND had a floor....
Now we do not have a "verb" for "congesta wagons".. As "and" is a parallel marker we want a clause to parallel a clause ..instead we get a noun "And" a clause... This is wrong !!

Process :
One error elimination (dont try to find all errors while solving...do this after you have solved the probelm for extra practie)

A) 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) wagons, with high wheels capable of crossing rutted roads, muddy flats, and the nonroads of the prairie, and with a floor that was curved upward at both ends to prevent
- This is even more horrible than A ... Here There is no Subject+verb pair (no clause) ..we have :Subject (modifier )..)modifier)...NO VERB

C) 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
- Same as A.. NO VERB ...



E) 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
- Multiple errors :
1. " and floors" ..by not inroducing a "Comma" , we here get a false parallelism scenario .. without a comma the structure might imply that "non roads" and "floors" are one entity included in the "roads" list...
2. "it prevented" : the pronoun "it" does not agree with plural noun "floors"..also here i think "it" is not at all an appropriat use becasue in option D we are elegantly told why the floors are curved ( to prevent) ..to+verb is usually used to tell the function of some entity !!


Takeaway :
1. "capable of" is correct idiom
2. Always make sure that the elements in the list make sense , and later check for parallelism (false and true)
3. discard the modifiers to find the MAIN VERB . In lengthy SC sentence the GMAT recurringly uses the pattern of giving an answer choice with NO S+V.
4. Two or more elements are present in a list, make sure the "And" before the last element of that list is preceded by a comma (VERY IMPORTANT)