Fossil fuels are actually slowly forming continuously, but we are using them up 15 approximately 100,000 times faster than they are formed.
答案：(C) at a rate
at a rate（of）表示「以…的速率」，後接倍數比較（100,000 times faster），為固定搭配；in effect（實際上）、by all means（務必、儘管）、on the point of（即將）皆不符合表示速率的語意與句構。
本題大考中心未公布逐題答對率，以下不談數據，只談觀念。
四個都是幾個字合起來當一個介系詞用（劍橋 English Grammar Today 的 Prepositions 條目），一個一個看都很眼熟，很容易憑語感挑。in effect（實際上）尤其誘人，因為前半句剛說化石燃料其實一直在慢慢形成，讀起來像要接一個「實際上」。
看空格後面接的是什麼：approximately 100,000 times faster than they are formed（比它們形成的速度快大約十萬倍）。那是一個速度，所以空格要填的必須是能帶出速度的說法，四個選項裡只有 at a rate 是講速率的（at a rate of ＋ 數量）。整句的對等連接詞是 but，它連的是兩個子句——前面說「其實一直在慢慢形成」、後面說「我們用掉的速度快十萬倍」，一慢一快正好對立，這也再次確認後半句的重點在速度。
in effect（實際上、事實上）是副詞片語，後面不接名詞，直接放進去會讓 100,000 times faster 整串懸空。by all means（當然可以、務必）用在答應對方的請求或加強語氣，與速度無關。on the point of 後面要接動名詞，意思是「正要做某事」（be on the point of leaving），既接不上後面的比較級，意思也不對。
at a rate of（以……的速率）、in effect（實際上、事實上）、by all means（當然、務必）、be on the point of V-ing（正要做某事）、use up（用光）、times faster than（比……快幾倍）
Modern societies have become increasingly vulnerable in their dependence on fossil fuels such as coal and petroleum. One 11 of using fossil fuels is that it can generate large amounts of energy by the simple process of burning. 12 , fossil fuels are readily available and are currently plentiful.
However, the burning of fossil fuels leads to the release of pollution into the atmosphere. Dependence on fossil fuels from unstable regions or countries creates energy security risks for dependent countries. Oil dependence 13 has led to monopolization, war, and socio-political instability.
Most importantly, fossil fuels are considered non-renewable resources, which will eventually decline in production and become 14 , with serious consequences to societies that remain highly dependent on them. Fossil fuels are actually slowly forming continuously, but we are using them up 15 approximately 100,000 times faster than they are formed.