an SAT writing problem

Question

Born in Italy in 1853, Maria Spelterini emigrated to the United States as a young woman and quickly became known for her breathtaking stunts. In 1876, the 23-year-old Spelterini became the only woman ever to cross the Niagara Gorge over a period of 18 days on a tightrope.

A) No change
B) on a tightrope, accomplishing that feat over a period of 18 days.
C) over a period of 18 days, she did this on a tightrope.
D) over a period of 18 days and, furthermore, doing this on a tightrope.

I had thought it would be A), but the answer is B). I think both works, but why would A) be wrong?

Answer 1

B) on a tightrope, accomplishing that feat over a period of 18 days.

The "breathtaking stunt" she accomplished (I believe you misquoted as students) was crossing Niagara Falls on a tightrope. How long it took is additional information.

It is unclear whether she remained on the tightrope for 18 days, crossed back and forth repeatedly over a period of 18 days, or finally succeeded on her 18th day of trying.