如果只係compare P同Q, 我可以argue tube Q之所以negative result, 純粹係因為冇amylase, 而唔係因為冇action of amylase on starch, 因此不能conclude action of amylase on starch
你需要有埋tube R, 去話比人聽即使有amylase存在(但冇starch), 嗰result依然係negative, 從而證明positive result in tube P係因為action of amylase on starch, 而唔係因為presense of amylase alone都可以產生positive result