单选题 When a psychologist does a general experiment about the human mind, he selects people ______ and asks them questions.
A. at ease B. at random C. in essence D. in sum

【正确答案】 B
【答案解析】本题的考点是固定词组的上下文辨析。at random意为“胡乱地，随便地”；根据上下文，选项B为正确答案。