______叫总功，计算公式为______．

Why have the Americans developed space shuttle according to the passage

Which of the following statements is NOT true according to the passage

A．The term "averted vision" refers to the act of looking out of the comers of one’s eyes.
B．If you want to see very faint things, you should better look sideways rather than straight on.
C．The receptors located in the eye’s center are less sensitive to dim light.
D．To "look" is to see something straight ahead.