The speed limit for bicycles on campus is 15 mph, otherwise posted.A. unlessB. untilC. s

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(答案→)A. unless
B. until
C. since
D. however
解析：句意：在校园内自行车的限速是每小时15英里，除非另外说明。这里用unless“除非，如果不”，until“直到”，since“自从”，however“然而”，所以选A。