问题

The scientists all over the world are trying their best to develop _____ energy, such as sunlight, wind and water for power and fuel. [ ]

A: unique
B: alternative
C: primary
D: instant

正确答案是：

同类题型

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We arrived at the station too early and had to go, so we sat there and chatted with each other.

Susan, whatare the advantages of MP5 players?—Mom, they aresmaller and lighter ______ they can be carried very easily.

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