Question

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

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

Answer 1

The sentence in which the adverbial clause modifies an adverb is The Double Eagle II was a hot-air balloon that flew farther (than other hot-air balloons.)
The adverb it modifies is farther.
All of the other sentences have clauses that modify verbs, not adverbs.