The size of the food they are about to eat is magnified while 27 of their hand remains normal.
答案：(B) that
that 在此代替前面出現過的單數名詞 the size，避免重複：食物的尺寸被放大，而手的尺寸維持不變；this 不用於這種比較替代，some 與 other 都無法代替 the size。
that 你大概只想到關係代名詞或「那個」，不知道它可以代替前面出現過的名詞，避免重複。本句完整寫法是 …while the size of their hand remains normal.，為避免名詞重複，the size 用 that 代替。
這一句完整寫出來是 …while the size of their hand remains normal.，前半句已經出現過 the size。英文語法習慣精簡，避免重複前面出現過的名詞，所以這裡用 that 代替前面出現過的單數名詞 size；如果前面出現過的是複數名詞，就用 those 代替。The population of Taipei is larger than that of Tainan.；The features of this phone are better than those of [the features of] older models.
that of／those of（代替前文出現過的名詞）、the same as that of（與…的相同）、compared with that of（與…的相比）。
Researchers from a university in Japan have developed "diet-glasses" that play tricks with one's perception of food, which could be helpful to people on a diet.
A camera and a viewing system are built 26 these glasses. As the wearer brings the food toward their mouth, the camera captures an image of the food. It replays the image back through the glasses after processing it on an attached computer. The size of the food they are about to eat is magnified while 27 of their hand remains normal. Their brain is 28 tricked into thinking they are eating more than they really are. The study showed that participants who wore the glasses ate 9.3% less than those who did not wear them.
The team has also developed a special device which uses scent bottles and visual trickery to make the wearers of these glasses think that the plain snack they are eating is 29 than it actually is. The device, for instance, can be set to 30 one's favorite flavor. Recent experiments with this device showed that 80% of the participants were fooled by the smell. For example, some participants thought that they were eating a chocolate snack, but in fact they were not.