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.

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.

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).