Mark‌ ‌the‌ ‌letter‌ ‌A,‌ ‌B,‌ ‌C,‌ ‌or‌ ‌D‌ ‌on‌ ‌your‌ ‌answer‌ ‌sheet‌ ‌to‌ ‌indicate‌ ‌the‌ ‌sentence‌ ‌that‌ ‌best‌ ‌combines‌ ‌each‌ ‌pair‌ ‌of‌ ‌sentences‌ ‌of‌ ‌the‌ ‌following‌ ‌questions: 

All possible means have been used to prevent air pollution. The sky is still not clear.

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