This “Charlie Brown effect” will be more 19 when the astronauts go on their Mars missions, which will take about three years to complete, much longer than missions to the moon.
答案：(C) severe
語意「這個『查理布朗效應』在火星任務時會更『嚴重』」，severe（嚴重的）符合，呼應前文說火星任務時間更長（三年）；contagious（傳染性的）、unusual（不尋常的）、aggressive（侵略性的）都不合。
This "Charlie Brown effect" will be more ___，四個形容詞接在 more 後面都成立。你可能被 contagious（有傳染性的）吸引，因為前面出現了 sickness、health risk，感覺跟疾病有關。但「查理布朗效應」是失重造成的生理反應，不是病菌引起的，不會傳染——跟症狀有關的字，不等於跟傳染有關的字。
關鍵在同一句的後半：when the astronauts go on their Mars missions, which will take about three years to complete, much longer than missions to the moon（火星任務約需三年，比登月任務長得多）。任務時間更長 → 處在失重狀態更久 → 症狀更嚴重，這條因果線只支持 severe（嚴重的），more severe 就是「更嚴重」。
contagious 需要有傳染源與傳染途徑，文章完全沒提；unusual（不尋常的）講的是罕見程度，任務變長不會讓這個現象變得比較罕見；aggressive 用在疾病上指「侵襲性強、惡化快」，通常形容癌症或治療方式，用來形容失重造成的臉部浮腫並不恰當。
severe symptoms／damage（嚴重的症狀／損害）、severe weather（惡劣的天氣）、a severe case of（嚴重的……病例）、contagious disease（傳染病）、be at risk of（有……的風險）、health risk（健康風險）。
Gravity has been at the top of the science agenda since the start of Mars missions. In the earlier days of space travel, scientists tried to overcome the force of gravity so that a rocket could shoot 16 Earth's pull in order to land humans on the moon. Today, they are more interested in how reduced gravity affects the 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 they are 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 "Charlie Brown effect" will be more 19 when the astronauts go on their Mars missions, which will take about three years to complete, much longer than missions to the moon. Moreover, the effect is often 20 space motion 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.