单选题 Social institutions are now being called ______ to provide assistance to the homeless.
【正确答案】 C
【答案解析】此题考查动词词组。call upon意为“号召，拜访”；call up意为“召唤，使想起，提出，打电话给”；call in意为“召集，召来，来访”；call off意为“叫走，放弃，使转移走，依次叫名”。