On what sentence does the underlined adverb clause modify an adverb?

a.) three men crossed the atlantic ocean in the double eagle 2 in 1978.

b.) some balloonist who had trued to cross the atlantic ocean disappeared before they were able to land.

c.) the double eagle 2 was a hot-air balloon that flew farther then other hot air balloons.

d.) when the double eagle 2 landed in paris, other balloonists who planned to cross the atlantic cancelled their trip.