Because so many visitors are visiting our school today, some teachers have to guides.
A. serve as
B. regard as
C. treat as
D. use as
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(答案→)A. serve as
B. regard as
C. treat as
D. use as
解析：词义辨析. A. 担任;充当 B. 把……认着 C. 对待；把……看做 D. 把……当做……使用。句意：因为今天有如此多的参观者来我们学校参观，一些老师不得不充当向导。选A。