Concept:Sentence rearrangement based on logical flow and connecting words.
Explanation:First, identify the part that introduces the main idea. Here, part P starts with "I realize" and states the topic of solving the climate change problem. It is the natural opening.
Part R follows because it continues the thought by stating that this task "will be much harder".
Part Q begins with "than", which directly links to the comparative word "harder" in part R. So Q must come after R.
Part S concludes by naming what the comparison is with: "the ozone depletion problem". This completes the sentence logically.
The correct sequence is P R Q S, forming: "I realize that solving the climate change problem will be much harder than solving the ozone depletion problem."
The other options do not maintain this logical flow.
Answer:Option A: P R Q S