单选题 The search was ______ when the fog got thicker.
A. called off B. called on C. called in D. called for

【正确答案】 A
【答案解析】本题考查动词短语。call off“取消叫走”；call on“号召，呼吁；邀请；访问”；call in“召集，召来；来访”；call for“要求；提倡；为…叫喊”。句意为：雾变得越来越大导致调查取消了。