6The world’s population is one of the biggest_________(challenge) of the world today.

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6Theworld’spopulationisoneofthebiggest_________(challenge)oftheworldtoday.6Theworld’spopulationisone

6The world’s population is one of the biggest_________(challenge) of the world today.
6The world’s population is one of the biggest_________(challenge) of the world today.

6The world’s population is one of the biggest_________(challenge) of the world today.
Challenges 加上s.以后要知道了,凡是one of...后面的东西是复数形式.