You are given a paragraph, with numbered sentences, and a question. Write the numbers of the sentences needed to answer the given question. Separate multiple sentence numbers with a comma (", "). E.g. "1, 2" would be the correct response if Sent 1 and Sent 2 are needed to answer the given question (do not include sentences that contain no information that is necessary to answer the question). Sort the sentence numbers in an increasing order (i.e., '1, 2', instead of '2, 1'). Your answer should not contain anything than the sentence numbers separated by a comma (e.g. '1, 2, 3, 4'). The given question requires more than one sentence to answer. So, your answer must contain at least one comma (", ").

Paragraph- Sent 1: Nathan read the package of words in silence, his only motions the steady progress of his eyes and occasional replacement of pages.
Sent 2: Allan sat nervously across from him in a chair Nathan had probably upholstered himself, a patchwork design of fabric containing easily more stuffing than any other furniture item of the period.
Sent 3: At long last, Nathan reached the end and set down his reading on the table between them.
Sent 4: Allan leaned forward unconsciously.
Sent 5: "It's the best story you've ever written."Sent 6: Allan exhaled and leaned back into the chair, his face relaxing in imitation of his thoughts.
Sent 7: "So," he asked, "you don't think it's a waste of ink and paper, a futile expedition into morbidity or literary debauchery?"Sent 8: "Heavens, no," said Nathan, aghast.
Sent 9: "This is one of the strongest works I've read in ages.
Sent 10: It speaks to the deepest storyteller's instinct within us all, yet is entirely original.
Sent 11: My dear friend, you have done it.
Sent 12: Oh, they may rail against you at first; they may decry you as a heathen or a literary savage; but while those in power say such things, others will read your tales and see their true worth.
Sent 13: Believe me when I say that you will be read a century from now."Sent 14: Allan, though dubious as to that possibility, felt some temptation from the compliment; mainly, it granted him the encouragement he still needed.
Sent 15: Nathan promised to show the story to a printer he knew and Allan left it with him, then walked home under the spreading maples with a smile lingering on his face.
Sent 16: He felt now that perhaps Nathan was right; although the man was somewhat peculiar, he had both an unimpeachable honesty and a certain propensity for insight.
Sent 17: Certainly, it was undeniable that the stories had an originality to them.
Sent 18: His mind's strangest fruit had ripened at last, and he found the taste less bitter than expected.
Sent 19: These thoughts and others like them filled his head as he walked the long road home.
Sent 20: It was evening, and the sky burned orange in the west when he neared home at last.
Question: why did Allan let Nathan take the story?.
15, 16.