91.There are still many problems_____before we are ready for a long stay on the moon.
答案解析
解析：
非谓语动词作后置定语的规则:现在分词表示主动或进行;过去分词表示被动或完成;不定式表示将
要发生:此处表示“要去解決的问题”,所以应用表将来的不定式。另外,many problems 和 solve 之问为迎镇上的动宾关系,因此应用不定式的被动式,即“to be+过去分词”的形式。故选D。
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