In contrast, fossil fuels, such as oil or coal, release 14 substances into the air when they are burned.
答案：(B) harmful
harmful substances（有害物質）表示化石燃料燃燒時會釋放有害物質到空氣中，與前文太陽能是「乾淨能源」形成對比；diligent／usable／changeable 均不符合此處語意。
四個選項都是形容詞，都能修飾後面的 substances，你會憑中文哪個順挑一個。
這是邏輯勾環，而且線索一定要跨句才看得到。句首的 In contrast（相較之下）就是那個勾環——它要求前後兩句站在相反的位置。前一句是 To begin with, it is a clean fuel（首先，太陽能是乾淨的燃料）；既然 In contrast 把方向翻過來，這一句講化石燃料燃燒後排出的東西，就必須是「不乾淨」的那一邊，也就是 harmful（有害的）。填進去讀成 release harmful substances into the air（把有害物質排放到空氣中）。
usable（可用的）與 clean 站在同一邊，被 In contrast 直接排除；changeable（易變的）講的是會不會變化，與乾不乾淨無關；diligent（勤奮的）只用在人身上，物質不會勤奮。三個都是形容詞，形態吻合，容易被順手選走。
harmful（有害的）、be harmful to（對……有害）、in contrast（相較之下）、release A into the air（把 A 排放到空氣中）、fossil fuels（化石燃料）、usable（可用的）。
The sun is an extraordinarily powerful source of energy. In fact, the Earth 11 20,000 times more energy from the sun than we currently use. If we used more of this source of heat and light, it 12 all the power needed throughout the world.
We can harness energy from the sun, or solar energy, in many ways. For instance, many satellites in space are equipped with large panels whose solar cells transform sunlight directly 13 electric power. These panels are covered with glass and are painted black inside to absorb as much heat as possible.
Solar energy has a lot to offer. To begin with, it is a clean fuel. In contrast, fossil fuels, such as oil or coal, release 14 substances into the air when they are burned. 15, fossil fuels will run out, but solar energy will continue to reach the Earth long after the last coal has been mined and the last oil well has run dry.