We are working on the _assumption that everyone will turn up.
A. assumption; invited
B. regulation; inviting
C. constitution; invited
D. expectation; inviting
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(答案→)A. assumption; invited
B. regulation; inviting
C. constitution; invited
D. expectation; inviting
解析：考查名词辨析和非谓语动词。句意“我们正在研究被邀请的每个人都会来的这样一个假设。”assumption“假设”；regulation“规则”；constitution“章程”；expectation“期望”；everyone和invite之间是被动关系，即“被邀请”，这里用过去分词作后置定语，相当于who is invited。故选A。