Choose the word or phrase among A, B, C or D that best fits the blank space in the following passage.
Gravity and weightlessness
Our everyday lives ___(1)___ such activities as sitting, walking, picking up things from the ground and lying in bed. None of these activities are possible in orbit.
___(2)___ a spacecraft reaches orbit, everything inside it seems to be weightless. Anything or anyone that is not tied down will float. Weightlessness allows astronauts to appear very ___(3)___. They can lift objects that would be ___(4)___ heavy to move on Earth. But there are some drawbacks.
___(5)___ the effect of gravity, blood and other body fluids begin to flow towards the head. This can cause a feeling of headaches. With no gravity to push against, bones and muscles can become weak. To ___(6)___ fit, they have to exercise several hours each day. This allows them to recover more quickly when they ___(7)___ to Earth.
In a space shuttle or space station, there is no up or down. There is no difference ___(8)___ a floor and a ceiling. This can make astronauts feel ___(9)___ until they get used to this strange ___(10)___.