Step 1: Read the passage once fully and underline the key fact: only about 3% of Earth's water is fresh. Step 2: The passage adds that most of this fresh water is locked as ice, so even less is easily usable. Step 3: The last sentence gives the conclusion: "therefore" every drop must be used wisely. So the reason is the scarcity of fresh water. Step 4: Eliminate wrong options: cost, total freezing and "no rain" are never mentioned in the passage. Answer: Because only a small part of Earth's water is fresh and usable.