OG8 阅读 Q47

Choice D is the best answer. The third paragraph explains that touching a single trigger hair results in "a rapid increase in the concentration of calcium ions" in the plant. The fourth paragraph further explains that the calcium concentration produced by this initial touch isn't enough to cause the trap to close, but that a second hair touch will bring the total concentration to the level necessary to close the trap: "a second hair needs to be stimulated to push the calcium concentration over this threshold and spring the trap."
Choices A and B are incorrect because the fourth paragraph explains that the second trigger supplements the action of the first trigger, not that it reverses it (choice A) or stabilizes its effect (choice B). Choice C is incorrect because the third paragraph clearly states that the calcium channels open after the first trigger hair is touched, not the second.