[单选题]下面关于极坐标系不正确的是（）
A.极坐标系由极点、极轴和极径组成
B.极坐标系角度取逆时针方向为正
C.椭圆、双曲线和抛物线这3种不同的圆锥截线，可以用一个统一的极坐标方程表示
D.极坐标系到直角坐标系的转化：y=ρcosθ,x=ρsinθ

The normal human daily cycle of activity is of some 7-8 hours’ sleep alternation with some 16-17 hours’ wakefulness and that the sleep normally coincides (1)_______the hours of darkness. Our present concern is with how easily and to what extent this (2)_______can be modified. The question is no mere academic one. The ease with which people can change from working in the day to working at night is a (3)______of growing importance in industry where automation (4)_______round-the-clock working of machines. It normally (5)______from five days to one week for a person to adapt to a (6)_______routine of sleep and wakefulness, sleeping during the day and working at night. (7)_______, it is often the case in industry that shifts are changed every week. This means that no sooner has he got used to one routine (8)_______ he has to change to another, (9)_______much of his time is spent neither working nor sleeping very (10)_______. One answer would seem to be (11)_______periods
A. shorter
B. better
C. longer
D. nicer

The person who can see a ship without some feeling of excitement must have very little imagination. Even the idea of leaving the solid land (1) most of us were born and brought (2) and going out on to the ever-moving waters must rouse (3) some feelings of strangeness. We may remember stories of terrible storms, with waves (4) mountains, and of people from ships which have sunk (5) weeks in small boats hundreds of miles from land. But we have also (6) joy of travelling on calm seas under blue (7) and of the (8) excitement of coming to a new beautiful land which we have seen only in pictures before.
(9) ships are not, of course, made chiefly for pleasure: their biggest use is in carrying goods from country to country. (10) ships can carry more goods than (11) means of transport, and can (12) so more cheaply. If ships (13) the British government would not be able to feed (14) A. Which
B. It
C. This
D. As