Others 17 the size of a small stone.
答案：(A) may be
may be 表示對現在事實的推測可能性，符合「其他流星體可能只有小石頭大小」的客觀描述語氣；would be 通常需搭配假設語境；must have been／could have been 為完成式，用於對過去事件的推測，與此處描述客觀現況不符。
四個選項都是助動詞加上 be，中間還混了兩個 have been，很容易一頭栽進「這是在考語氣還是在考時態」的分析。這一題不必想那麼遠，前一句就把答案定下來了。
把前後兩句擺在一起看：Some are very large and may be hundreds of feet wide. Others ___ the size of a small stone.（有些非常大，可能寬達數百英尺。其他的只有小石頭那麼大。）Some... Others... 是一組成對的說法，兩句在講同一件事的兩半，句型與時態理當一致。前一句用的是現在式的 are 與 may be，講的是流星體現在普遍的樣子；後一句照抄同一個形態就對了。這一招在克漏字裡非常好用：遇到並列或對稱的結構，先找出前面那一個是什麼形態，後面照抄。
must have been 與 could have been 都是助動詞加上 have 與過去分詞，講的是對過去的推測（當時一定是……、當時可能是……）。但本文在描述流星體現在的性質，不是在回推某件已經發生的事，時間層整個對不上。would be 用在假設或過去的未來，本句既沒有假設的前提，也沒有一個過去的時間點可以往後看。鑑別度 56，分開的是有沒有回頭讀前一句，還是只盯著空格附近推敲語氣。
Some... Others...（有些……其他的……）、may be（可能是，表推測）、must have ＋ 過去分詞（當時一定……）、could have ＋ 過去分詞（當時可能……）、the size of（……那麼大）、chunks of rock or metal（石塊或金屬碎塊）
Do you know the difference between the terms meteoroid, meteor, and meteorite in astronomy? Many people find these words confusing. However, the difference is all about their 16.
Meteoroids are far up in the sky. They are chunks of rock or metal that speed through space. Some are very large and may be hundreds of feet wide. Others 17 the size of a small stone. Most meteoroids travel around the sun in space and stay away from the Earth. However, sometimes a meteoroid will enter the Earth's atmosphere. Friction with the atmosphere will cause it to 18 and burn while traveling at high speed.
As a meteoroid begins to burn in the atmosphere, it leaves a streak of light. When this tail-like light is falling down toward the Earth, it is called a meteor, or a shooting star. Most meteors vaporize completely before they hit the ground.
If any meteor 19 its fiery journey through the atmosphere and lands on Earth, it is called a meteorite. Large meteorites can cause great explosions and much destruction on the surface of the Earth. 20 , Barringer Crater in the American state of Arizona, measuring 1,200 m in diameter and some 170 m deep, was produced by a meteorite impact.