Your brain doesn’t know whether you are stationary or moving when these parts send conflicting information: One part of your balance-sensing system detects that your body is moving, but the other parts 18 .
答案：(C) don’t
don't 代替 don't detect that your body is moving（省略重複部分），表示「平衡感應系統的一部分偵測到身體在動，但其他部分『沒有』」；aren't（be動詞否定，前面用的是一般動詞 detect不是be動詞）、won't（未來式否定）、haven't（現在完成式否定，前面是現在簡單式detects）都不合。
空格只有一個否定的助動詞，後面什麼都沒接，你會覺得句子好像沒寫完，於是憑感覺挑一個。其實這是英文很常見的「替代」用法（文法上叫 substitution）：用一個助動詞頂替前面出現過的整串動詞，避免重複。這個你國中就學過——Do you like it? — Yes, I do.，那個 do 代替的就是 like it。本句 but the other parts ___ 一模一樣，don't 代替的是 don't detect that your body is moving。
劍橋《English Grammar Today》的 Substitution 條目說明：do 可以用來代替一個動詞連同它後面所帶的成分；同一套文法書的 Ellipsis 條目把兩個概念分得很清楚——省略是「直接不寫出來」，替代是「用 do、so、not 這類字頂上去」。所以判斷只有一步：回頭找出它代替的是哪個動詞，時態與人稱就跟著那個動詞走。 前半句是 One part of your balance-sensing system detects...，動詞是一般動詞 detects、現在簡單式；空格的主詞 the other parts 是複數，所以用 don't。
aren't 用在 be 動詞，可是前面的動詞是 detect 不是 be；haven't 用在現在完成式，前面卻是 detects（現在簡單式），時態對不上；won't 是未來式，但文章描述的是暈動症發生當下的生理狀況，不是將來才會發生的事。四個選項的差別完全取決於前面那個動詞是什麼，先找到它，答案就定了。
Do you like it? — Yes, I do.（do 代替 like it）、So do I.（我也是）、Neither do I.（我也不是）、She liked it, but I didn't.（didn't 代替 didn't like it）、balance-sensing system（平衡感應系統）、detect（偵測）、conflicting（互相矛盾的）。
If you have ever been sick to your stomach on a rocking boat or a bumpy car ride, you know the discomfort of motion sickness. It can 16 suddenly, progressing from a feeling of uneasiness to a cold sweat. Soon, it can lead to dizziness, nausea, and vomiting.
Motion sickness occurs when the signals your brain receives from your eyes, ears, and body 17 . Your brain doesn't know whether you are stationary or moving when these parts send conflicting information: One part of your balance-sensing system detects that your body is moving, but the other parts 18 . Your brain's confused reaction makes you feel sick. You may experience the discomfort from the motion of cars, boats, or amusement park rides. You may also get sick from playing video games, or looking through a microscope.
You can take some 19 to help avoid the discomfort. If you are traveling, reserve seats where motion is felt the least, such as the front row of a car or forward cars of a train. Looking out into the distance from the vehicle can help 20 . You can also take medicine before your ride to avoid or reduce nausea and vomiting.