单选题 ______ at the outset, ______ instead of shifting things about may be pheromones released when they reach committee size.

【正确答案】

【答案解析】本题的重点在于理清句子结构。本句主语the stimuli同时也是定语从句that set them off at the outset的先行词，句子谓语是may be，而building collectively是分词短语作状语。