单选题 Planet "Y" orbits the Sun in six months. AB is four times the distance of CD and moreover it turns out that planet X travels at haft the speed of planet Y. How long does Planet "X" take to orbit the Sun?

A、 8 years.
B、 6 years.
C、 4 years.
D、 2 years.

【正确答案】 C

【答案解析】由题意可知AB=4CD，故x绕太阳一周的距离为Y的4倍，又planet X travels at half the speed of planet Y，故x绕太阳一周的时间为Y的8倍。Planet Y绕太阳一周的时间为6个月，故planet X绕太阳一周需4年。