**Core Concept:** Vocabulary - Synonym/Antonym辨析 **Analysis:** The word 'trigger' in scientific contexts means to initiate or activate a process. 'Activate' (B) directly matches this meaning. 'Prevent' (A) is an antonym, 'Reduce' (C) indicates decrease, and 'Observe' (D) refers to monitoring, not causation. **Key Point:** 'Trigger' implies causing a reaction to start. **Common Mistake:** Confusing 'trigger' with similar-sounding words like 'reduce' due to surface-level associations.