第 16 至 20 題為題組       Gravity has been at the top of the science agenda since the start of Mars missions. In the earlier daysof space travel, scientists tried to overcome the force of gravity so that a rocket could shoot  16  Earth’spull in order to land humans on the moon. Today, they are more interested in how reduced gravity affectsthe astronauts’  17  condition.      Our bodies have evolved to exist within Earth’s gravity (1 g), not in the weightlessness of space (0 g)or the microgravity of Mars (0.3 g). When on Earth, we have more fluids in our lower body because theyare pulled down by Earth’s gravity. However, with the absence of gravity in the outer space, our body fluids  18  , shifting toward the upper body and the head. As a result, the astronauts have swollen, puffy faces,very much resembling that of the round-headed Charlie Brown in the famous comic strip. This “CharlieBrown effect” will be more  19  when the astronauts go on their Mars missions, which will take aboutthree years to complete, much longer than missions to the moon. Moreover, the effect is often  20  spacemotion sickness, headaches, and nausea. Such a syndrome is considered the top health risk for the astronauts,and scientists are still trying to figure out how it may be prevented.
【題組】16.
(A) back to
(B) free of
(C) long before
(D) straight on