D

Now that man has actually landed on the suce of the moon, he has learned many new things about it. But one thing man knew before he ever reached the moon was that there was no life on it. There is no atmosphere on the moon. The lack of air means that the moon is not protected from any of the sun’s rays. The sun sends out heat and light radiation (辐射). Life on the earth depends on heat and light. But the sun also sends out erous kinds of radiation. The earth’s atmosphere protects us from most of them. On the moon, however, there is no atmosphere to stop the radiation. All the sun’s rays beat down on the suce of the moon. Because there is no atmosphere, the moon’s suce is either extremely hot or extremely cold. As the moon circles around the earth, the side of it that is lighted up by the sun becomes very hot. The temperature there reaches more than 300 degrees Fahrenheit (华氏温度). This is hotter than boiling water. The hot lunar day lasts two weeks. It is followed by a night that is also two weeks long. At night the temperature drops to about 260 degrees below zero Fahrenheit. "This is more than twice as cold as temperatures reached at the earth’s South Pole. Under these conditions, no form of life that we know of here on the earth could exist on the moon.

The underlined word "lunar" ( Line 4, Para. 3) probably means something about ______.