答案：AACBD
词汇讲解
1. caterpillar 链轨, 履带车; 坦克车; 履带拖拉机
A butterfly is produced by metamorphosis from a caterpillar.
蝴蝶是由毛虫蜕变而成的。
Jacob was a hairy man; a hairy caterpillar.
雅各是个多毛的男人；毛虫。
2. formidable 可怕的, 可畏的
To grapple with an unexpectedly formidable opponent.
遇上劲敌与一名出乎意料难对付的对手拼搏
The king's mother was formidable old lady.
国王的母亲是个令人望而生畏的老妇人。
3. inexhaustible 无穷尽的, 用不完的
I shall die again and again to know that life is inexhaustible.
我将死了又死，以明白生是无穷无尽的。
These guerrilla units are powerful auxiliaries to the main force and serve as inexhaustible reserves for its continuous growth.
这种广大的游击部队，造成这个主力的丰富的羽翼，又是这个主力继续扩大的不断的源泉。
4. insuperable 不能克服的, 难以超越的
Your 15th difficulty not insuperable expect your utmost endeavor advise result
15日电悉，困难并非不可克服，望尽最大努力并电告结果。