单项选择题
A time diagram is a diagram on the plane of the（）.
A.celestial meridian B.celestial equator C.celestial horizon D.principal vertical circle
单项选择题 A time diagram is a diagram of the celestial sphere as observed from above the（）.
单项选择题 A thrust block is designed to（）.
单项选择题 A thin，whitish，high cloud popularly known as “mares’ tails” is（）.
