The four sentences (labelled 1, 2, 3, 4) below, when properly sequenced would yield a coherent paragraph. Decide on the proper sequencing of the order of the sentences and key in the sequence of the four numbers as your answer:

1. But the attention of the layman, not surprisingly, has been captured by the atom bomb, although there is at least a chance that it may never be used again.
2. Of all the changes introduced by man into the household of nature, [controlled]large-scale nuclear fission is undoubtedly the most dangerous and most profound.
3. The danger to humanity created by the so-called peaceful uses of atomic energy may, however, be much greater.
4. The resultant ionizing radiation has become the most serious agent of pollution of the environment and the greatest threat to man’s survival on earth.

The correct sequence is 2 → 4 → 1 → 3.

Let us carefully analyze each sentence and understand the logical thread that connects them in this order:

Sentence 2 (Opening/Topic Sentence):
"Of all the changes introduced by man into the household of nature, large-scale nuclear fission is undoubtedly the most dangerous and most profound."
This sentence is the ideal opener because it introduces the central subject — large-scale nuclear fission — and sets the tone by declaring it the most dangerous change mankind has made to nature. It is broad, declarative, and scene-setting, which are the hallmarks of a good topic sentence. It tells us: here is the biggest threat.

Sentence 4 (Direct Consequence/Follow-up):
"The resultant ionizing radiation has become the most serious agent of pollution of the environment and the greatest threat to man's survival on earth."
The key word here is "resultant" — this word acts as a logical connector, referring directly back to the nuclear fission introduced in Sentence 2. It explains the consequence of that fission: ionizing radiation. It deepens and elaborates the danger mentioned in Sentence 2 by specifying the mechanism of harm (radiation, pollution, threat to survival). Sentence 4 must follow Sentence 2 because the word "resultant" only makes sense after a cause has been named.

Sentence 1 (Contrast — Public Perception vs. Reality):
"But the attention of the layman, not surprisingly, has been captured by the atom bomb, although there is at least a chance that it may never be used again."
The word "But" signals a contrast or a pivot. After Sentences 2 and 4 have established that nuclear fission and its radiation pose the greatest environmental danger, Sentence 1 points out that the general public (the layman) is focused on the atom bomb — a more visible, dramatic threat — not on the subtler, ongoing danger of radiation and peaceful nuclear use. The phrase "although there is at least a chance that it may never be used again" hints that the bomb may not be the real long-term problem, gently nudging the reader toward the real concern.

Sentence 3 (Closing Twist — The True Danger):
"The danger to humanity created by the so-called peaceful uses of atomic energy may, however, be much greater."
This is the concluding twist of the paragraph. The word "however" introduces a final contrast: while the layman worries about the atom bomb (Sentence 1), the author argues that the so-called peaceful uses of atomic energy — the nuclear power plants, reactors, and radiation sources not used as weapons — may actually pose an even greater danger. This rounds off the paragraph beautifully by redirecting the reader's concern from the dramatic (the bomb) to the insidious (peaceful nuclear applications).

Summary of the Logical Flow:

Sentence 2 → Introduces nuclear fission as the most dangerous change.
Sentence 4 → Explains the resultant danger: ionizing radiation and environmental threat.
Sentence 1 → Contrasts this with the public's fixation on the atom bomb, which may never be used.
Sentence 3 → Delivers the final irony: the "peaceful" uses of atomic energy may be even more dangerous than the bomb.

The paragraph moves from a general claim → specific consequence → common misconception → surprising truth, making 2413 the only sequence that produces a coherent and logically progressive paragraph.