Why is this group of words not a sentence?

you cannot start a sentence with a question

Explanation:

this is b. how railways dealt with people riding the trains without paying

ich statement best shows a problem in travels with charley?

one sharp difficulty presented itself. in the intervening twenty-five years my name had become reasonably well known.

this being so, my trip demanded that i leave my name and my identity at home.

i had to go alone and i had to be self-contained, a kind of casual turtle carrying his house on his back.

with all this in mind i wrote to the head office of a great corporation which manufactures trucks.