There is a sentence that is missing in the paragraph below. Look at the paragraph and decide where (option 1, 2, 3, or 4) the following sentence would best fit.
Paragraph: Early in this century a young Breton schoolboy who preparing himself for a scientific career began to notice a strange fact about hens in his father's poultry yard ___ (1) ___ . As they scratched the soil they constantly seemed to be pecking at specks of mica, a siliceous material dotting the ground ___ (2) ___ . No one could explain to Louis Kervran why the chickens selected the mica, or why each time a bird was killed for the family cooking pot no trace of the mica could be found in its gizzard ___ (3) ___ . It took Kervran many years to establish that the chickens were transmuting one element into another ___ (4) ___ .
Sentence: Yet each day the flock produced eggs with calcareous shells though they apparently had not ingested any calcium from land which was entirely lacking in limestone.
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