**Core Concept:** Grammar - Clause Relationships **Analysis:** 'Because' (A) correctly shows cause-effect: more chemicals cause higher responses. 'Therefore' (B) needs preceding cause. 'Although' (C) contradicts logic, and 'Meanwhile' (D) indicates time, not causation. **Key Point:** Causal links require 'because'. **Common Mistake:** Confusing 'because' with 'therefore' due to similar causal roles.