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

A、 The stimuli set them off...build collectively
B、 The stimuli that set them off...building collectively
C、 The stimuli setting them off...they build collectively
D、 Being set off by stimuli...their building collectively

【正确答案】 B

【答案解析】解析：分析句子，主语应是stimuli，谓语部分是“may be pheromones released”。第一个空格处主语后跟定语从句that set them off at the outset，第二空格处为分词短语building collectively做状语，句子整体表达通畅，故选B。