A．They are found under the oceans. B．They were most active when earth was first formed. C．Their emissions created earth's atmosphere. D．Their fumes are mostly water in the state of a gas.
A.They B. B．They C. C．Their D. D．Their
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D 解析：信息明示题，女士指出，地质学者认为火山气体大多为蒸汽，故选D，考点 fill sth. up 意为“填......