Explanation:

- The passage discusses the creation of urban forests to combat air pollution.

- It mentions involving people in this process by planting seeds or creating seed bombs.

- The need for authorities to allocate space for planting trees is emphasized.

- The last blank requires a word to indicate what planting trees provides to people.

Option Analysis:

- Option 1: Availability – This word implies that the act of planting trees provides a sufficient amount of oxygen.

- Option 2: Shortage – This suggests a lack of oxygen, which contradicts the purpose of planting trees.

- Option 3: Lack – Similar to shortage, it implies a deficiency, which does not fit the context.

- Option 4: Mix – This word does not fit contextually with providing oxygen.

The correct word is related to ensuring a supply of oxygen, so:

- Correct Answer: Option 1 – Availability

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