【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