Besides, cash crops and tourism are also 29.
答案：(A) significant
significant 意為「重要的、可觀的」：除了水力發電，經濟作物與觀光收入也相當可觀；durable（耐用的）、frequent（頻繁的）、expressive（有表現力的）都無法形容收入來源。
答對率 38%。
四個形容詞看起來都能填，但要靠前一句才決定得了方向。
前一句說 these less than 50% of the GDP now that Bhutan has become an exporter of hydroelectricity（既然不丹已成為水力發電出口國，這些收入已占不到 GDP 的一半）——言下之意是水力發電已經是重要收入來源。本句以 Besides 起頭、句中又有 also，就是在「再加上一項同樣重要的」：經濟作物與觀光。沒有前一句，「也很…」就沒有比較的對象，所以這一格必須跨句判斷。「看到 also、besides、too，先回頭找它要對照的是什麼。」
durable（耐用的）用於物品；frequent（頻繁的）形容次數；expressive（有表現力的）形容表情或作品。三者都不用來形容收入來源。
be significant（重要、可觀）、a significant amount／impact（可觀的數量／重大的影響）、account for（占…比例）、an exporter of（…的出口國）。
The Kingdom of Bhutan is a landlocked nation situated between India and China. The entire country is mountainous, 26 a small strip of subtropical plains in the extreme south. The elevation gain from the subtropical plains to the glacier-covered Himalayan heights exceeds 7,000 m.
Its traditional 27 is based on forestry, animal husbandry and subsistence agriculture. However, these 28 less than 50 of the GDP now that Bhutan has become an exporter of hydroelectricity. Besides, cash crops and tourism are also 29.
Bhutan is one of the most isolated nations in the world, 30 foreign influences and tourism regulated by the government to preserve its traditional Tibetan Buddhist culture. It is often described as the last surviving refuge of traditional Himalayan Buddhist culture.