During the 1920s–1950s, which finding supported continental drift?

• A. Seafloor mapping discovered mountain ranges & trenches running parallel to continental edges
• B. Fossil distribution across continents showed identical species on all continents
• C. Atmospheric CO2 levels were rising globally
• D. Ocean temperatures were uniform

Answer: (A)

The key idea being tested is how we recognize evidence that the Earth's crust is in motion as a system of moving plates. Seafloor mapping revealing mountain ranges and trenches that run parallel to continental edges provides a direct clue that large slabs of crust are interacting at boundaries. Trenches mark where one plate sinks beneath another, and ridges show where new crust forms as plates pull apart, all of which fits the picture of crustal movement that drives continents to drift over time. This kind of ocean-floor evidence connects with the geographic fit of continents and with how the planet’s outer shell behaves as a set of interlocking plates.

Fossil distribution across continents does support the idea, since similar species appear on continents that are now far apart, suggesting they were once joined. But the statement about identical species on all continents is too strong and not what was observed. The other two options— atmospheric CO2 levels rising globally and uniform ocean temperatures—don’t address the movement of the crust at all.