【单选题】

According to the passgae, the sentence "But if the earth…for optimum comfort" (in the 3rd paragraph) means
A．there is an unseen hand controlling the earth' s movement

B．there is an operating manual warning people against air-conditioning in the factory
C．people should prepare operating manuals to tell workers how to get greater comfort
D．people have been enjoying the most comfortable climate in Earth' s history.