In which sentence does the underlined adverb clause modify an adverb?


a.The Double Eagle II was a hot-air balloon that flew farther (than other hot-air balloons.)
b.(When the Double Eagle II landed in Paris), other balloonists who had planned to cross the Atlantic cancelled their trip.
c.Some balloonists who had tried to cross the Atlantic Ocean disappeared( before they were able to land.)
dThree men crossed the Atlantic Ocean (in the Double Eagle II) in 1978.

the things in parenthesis is the part suppsed to be underlined.