Atestable explanation that's supported or refuted is called a/an
a. experiment
b. reaction
c. hypothesis
d. variability

The answer is the option C. hypothesis. If the option A, confuses you, think that an experiment is not per se an explanation, it is just a way to run trials to determine something. Hypothesis are statements that establish a possible relations between facts but that are not proved. Then you can design an experiment to test the validity or not of the hypothesis. You should not make hypothesis that cannot be testable.

The soviet union beat the us to the first successful launch, with the october 4, 1957, orbiting of sputnik 1, and later beat the us to have the first human in earth orbit, yuri gagarin, on april 12, 1961. the "race" peaked with the july 20, 1969, us landing of the first humans on the moon with apollo 11