It may have something to do ______ our one-child family policy,I find young people today tend to keep to themselves.
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c______ adj. in a state of complete disorder and confusion
u______ n. the state of being the only one of its type
i______ v. to suggest that sth. is true without saying this directly
She followed the receptionist down a luxurious corridor to a closed door, ________ the woman gave a quick knock before opening it.
A. on which
B. but when
C. wherein
D. then