If we used more of this source of heat and light, it 12 all the power needed throughout the world.
答案：(D) could supply
與現在事實相反的假設語氣：If + 過去式（used）, 主要子句用 could/would + 原形動詞，could supply 符合這種假設語氣的公式。supplies（直述句現在式）、has supplied（現在完成式）、was supplying（過去進行式）都不符合假設語氣的動詞形式配對。
看到 If 你大概直接讀成「如果……就……」，順手填一個現在式進去；但這一句的 If 子句用的是過去式 used，那正是一個訊號。
鑑別度高達 58，分開的是有沒有先看 If 子句的動詞形態。If 子句寫的是 used（過去式），可是講的並不是過去的事——太陽現在就在那裡，是我們現在沒有多用它。這就是與現在事實相反的假設語氣，公式是 If ＋ 過去式，主要子句用 would／could／might ＋ 原形動詞。國中的掛鉤就是 If I were you, I would tell him.（我不可能是你，所以 were 配 would）。四個選項裡只有 could supply 符合這個形態，填進去讀成 it could supply all the power needed throughout the world（它就能供應全世界所需的電力）。
supplies 是現在式，會把整句讀成單純的條件句，與 If 子句的過去式對不上；has supplied 是現在完成式、was supplying 是過去進行式，兩個都在陳述已經發生或當時正在發生的事實，與「假設」剛好相反。三個都是 supply 的形態，形態吻合，容易被順手選走。
If ＋ 過去式，S ＋ would／could ＋ 原形動詞（與現在事實相反的假設語氣）、If I were you（如果我是你）、supply A with B（供應 A 以 B）、harness energy（利用、駕馭能源）、throughout the world（全世界）、all the power needed（所需的全部電力）。
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.