Consider the following events.1.First reptiles2.First insects3.Shelled animals4.First mammals. Arrange the above events in Chronological order of their origin on the earth andselect the correct answer from the codes given below: Codes: A B C D

1. A

   2 1 3 4

   2 1 3 4 would put first insects before shelled animals and reptiles before shells. Shelled marine animals appear in the Cambrian, before terrestrial insects. This order is too early for insects.

2. B

   2 3 1 4

   2 3 1 4 still starts with insects before shells. That inverts Cambrian shelly fauna and later insect origin. The first two slots are wrong.

3. C

   3 2 1 4

   (c) Official key: 3 2 1 4 is shelled animals, then first insects, then first reptiles, then first mammals. Cambrian shells precede land insects; insects precede reptiles; reptiles precede mammals. That is the stored chronological code.

4. D

   2 4 1 3

   2 4 1 3 puts mammals before reptiles and shells last, which reverses vertebrate history. Mammals cannot predate reptiles. The code is impossible.

Summary. Official key is (c). Earth-life order here is shells (Cambrian), insects, reptiles, mammals. Code 3-2-1-4 stores that sequence. Starting with insects or placing mammals early breaks the fossil calendar. Honour 3 2 1 4. The official chronology is shelled animals before the three later land groups.