Gaps in the treetops resulting from crown shyness may allow trees to increase their 14 light and enhance photosynthesis.
答案：(B) exposure to
【句型】exposure to 字詞搭配「暴露於/接觸到」，表示樹冠間的縫隙讓樹木能增加「接觸」陽光的機會；reliance on（依賴）、sensitivity to（對……敏感）、reflection on（對……的反思）都不合。
exposure 你最常在「曝光」「曝曬」的語境看到，感覺帶點負面（exposure to the sun 曬到太陽）。其實 exposure to sth 是中性的「接觸到、暴露於」，好壞由後面的名詞決定：exposure to sunlight 對植物是好事，increase their exposure to light 就是「增加接收到的光」。
整句是 Gaps in the treetops... may allow trees to increase their ___ light and enhance photosynthesis。and 後面的 enhance photosynthesis（促進光合作用）是關鍵：要行光合作用就得曬到光，所以空格講的是「接收到光的量」。四個選項都是「名詞＋介系詞」的組合，這種題型先確認介系詞、再確認語意；本題四個介系詞形式上都能接 light，就得靠語意定案。
reliance on（對……的依賴）語氣太重，樹木不是變得「更依賴」光，而是「接觸更多」光；sensitivity to（對……敏感）指的是反應強弱，不是接收量；reflection on 有「對……的反思」與「光的反射」兩義，但光被反射是彈開，跟增加吸收剛好相反。
exposure to sth（接觸、暴露於……）、be exposed to（接觸到……）、reliance on sth（對……的依賴）、rely on（依賴）、sensitivity to sth（對……敏感）、be sensitive to（對……敏感）、reflect on sth（反思……）。
In certain forests, when you look up you will see a network of cracks formed by gaps between the outermost edges of tree branches. It looks like a precisely engineered jigsaw puzzle, each branch growing just perfectly so it almost, 11 , touches the neighboring tree. This beautiful phenomenon is called crown shyness.
Scientists have been discussing crown shyness since the 1920s, proposing 12 explanations for the phenomenon. Some researchers point out that as trees often grow close together, treetops can easily collide and break when swayed by the wind. In order to protect their branches from breakage, trees maintain "shyness gaps"—spaces large enough to prevent them from touching their neighbors.
Other scientists suggest that plants, like animals, 13 resources—nutrients, water, space, and light. In forested areas with thick tree crowns, there is intense struggle for these resources. Gaps in the treetops resulting from crown shyness may allow trees to increase their 14 light and enhance photosynthesis. Additionally, by having branches that do not touch those of their neighbors, trees may be able to limit the spread of leaf-eating insects, and potentially also the transmission of diseases from tree to tree.
15 decades of investigation, there is no consensus on exactly what causes the beautiful and mysterious phenomenon of crown shyness.