Problem: Given the question: Extract the answer to the question from the following context. Question: What unknown other than heredity remained unresolved until the 1900s? Context: There were serious scientific objections to the process of natural selection as the key mechanism of evolution, including Karl von Nageli's insistence that a trivial characteristic with no adaptive advantage could not be developed by selection. Darwin conceded that these could be linked to adaptive characteristics. His estimate that the age of the Earth allowed gradual evolution was disputed by William Thomson (later awarded the title Lord Kelvin), who calculated that it had cooled in less than 100 million years. Darwin accepted blending inheritance, but Fleeming Jenkin calculated that as it mixed traits, natural selection could not accumulate useful traits. Darwin tried to meet these objections in the 5th edition. Mivart supported directed evolution, and compiled scientific and religious objections to natural selection. In response, Darwin made considerable changes to the sixth edition. The problems of the age of the Earth and heredity were only resolved in the 20th century.
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The answer is:
the age of the Earth

Problem: Given the question: Extract the answer to the question from the following context. Question: Ships are what vessels? Context: The French Marines and naval infantry intended for the invasion of northern Germany were dispatched to reinforce the French Army of Chalons and fell into captivity at Sedan along with Napoleon III. A shortage of officers, following the capture of most of the professional French army at the Siege of Metz and at the Battle of Sedan, led naval officers to be sent from their ships to command hastily assembled reservists of the Garde Mobile. As the autumn storms of the North Sea forced the return of more of the French ships, the blockade of the north German ports diminished and in September 1870 the French navy abandoned the blockade for the winter. The rest of the navy retired to ports along the English Channel and remained in port for the rest of the war.
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The answer is:
naval

Problem: Given the question: Extract the answer to the question from the following context. Question: How are tones normally played when determining jnd? Context: The just-noticeable difference (jnd) (the threshold at which a change is perceived) depends on the tone's frequency content. Below 500 Hz, the jnd is about 3 Hz for sine waves, and 1 Hz for complex tones; above 1000 Hz, the jnd for sine waves is about 0.6% (about 10 cents). The jnd is typically tested by playing two tones in quick succession with the listener asked if there was a difference in their pitches. The jnd becomes smaller if the two tones are played simultaneously as the listener is then able to discern beat frequencies. The total number of perceptible pitch steps in the range of human hearing is about 1,400; the total number of notes in the equal-tempered scale, from 16 to 16,000 Hz, is 120.
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The answer is:
quick succession