答案：AACBD

詞匯講解

1. caterpillar 鏈軌, 履帶車; 坦克車; 履帶拖拉機

A butterfly is produced by metamorphosis from a caterpillar.
蝴蝶是由毛蟲蛻變而成的。

Jacob was a hairy man; a hairy caterpillar.
雅各是個多毛的男人；毛蟲。

2. formidable 可怕的, 可畏的

To grapple with an unexpectedly formidable opponent.
遇上勁敵與一名出乎意料難對付的對手拼搏

The king's mother was formidable old lady.
國王的母親是個令人望而生畏的老婦人。

3. inexhaustible 無窮盡的, 用不完的

I shall die again and again to know that life is inexhaustible.
我將死了又死，以明白生是無窮無盡的。

These guerrilla units are powerful auxiliaries to the main force and serve as inexhaustible reserves for its continuous growth.
這種廣大的游擊部隊，造成這個主力的豐富的羽翼，又是這個主力繼續擴大的不斷的源泉。

4. insuperable 不能克服的, 難以超越的

Your 15th difficulty not insuperable expect your utmost endeavor advise result
15日電悉，困難并非不可克服，望盡最大努力并電告結果。