Sentence ordering (PQRS) — Arrange the parts to state that the appearance of the first mammals on the earth, at the time when the dinosaurs were at their peak, went almost unnoticed (detailed solution and reasoning).

Given data

• Lead (given): ‘‘The appearance …’’
• P: ‘‘this/the dinosaurs were at their peak’’
• Q: ‘‘of the first mammals on the earth’’
• R: ‘‘at the time when’’
• S: ‘‘went almost unnoticed’’

Concept/Approach
Complete the head noun ‘‘appearance’’ with the prepositional phrase (Q). Supply the time modifier (R + P). Conclude with the finite verb (S) to close the sentence.

Step-by-step construction
Attach Q: ‘‘The appearance of the first mammals on the earth …’’Add R: ‘‘… at the time when …’’Follow with P: ‘‘… the dinosaurs were at their peak …’’Finish with S: ‘‘… went almost unnoticed.’’Full sentence: ‘‘The appearance of the first mammals on the earth, at the time when the dinosaurs were at their peak, went almost unnoticed.’’

Why other orders fail
Starting with P or S yields fragments; the verb ‘‘went’’ (S) must close the subject string introduced by ‘‘The appearance …’’.

Final Answer
QRPS