1. He was fond of saying, "A foreign language is a weapon in the struggle of life."
    他常喜欢说：“外国语是人生斗争的一种武器。”

2. As is so often pointed out, knowledge is a two-edged weapon which can be used equally for good or evil.
    正像人们常常指出的，知识是一把双刃刀，可以用于造福，也可以用来为害。

3. Though it would be a two-edged weapon when they fought each other.
    不过，当它们相互撕打时，毒液就成了利弊参半的武器，可以杀死对方，也可以被对方的毒液杀死。

4. It´s a projectile weapon.
    这是件投掷式武器。