题目

When I was growing up, I had an old neighbor named Doctor Gibbs. He didn't look like any doctor I'd ever known. He never yelled at us for playing in his yard, but was always very kind. When Doctor Gibbs wasn't saving lives, he was planting trees. He had some interesting theories about planting trees. He believed in the principle: No pains, no gains. He hardly watered his new trees, which flew in the face of conventional wisdom. Once I asked why and he told me that watering plants spoiled them because it made them grow weaker. He said you had to make things tough for the trees so that on人体呼吸时，呼出的气体中二氧化碳比氧多．______．