Obsidianis(formed)(when)siliceouslava(co_考试试题_托福

【单选题】Obsidian is (formed) (when) siliceous lava (cools) too rapidly to (crystallized) into rock-fanning minerals.
A、formed
B．when
C．cools
D．crystallized

网考网参考答案：D
网考网解析：
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