单选题 Lincoln proclaimed that the problem of setting free the black slaves should ______ immediate solution.
A. call off B. call for C. call on D. call in

【正确答案】 B
【答案解析】[解析] call for“需要”，“提倡”，“号召”；call off“取消”，“撤销(计划的活动)”；call on“拜访”；call in“召集”。根据题意，B是正确答案。