题目内容

群G和{e}都是G的子群，称为G的子群。

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设是代数系统,∗,∘是二元运算。若∗和∘运算满足交换律、结合律、, 则构成格。

如果一个格有格，则该格为称为布尔格或布尔代数。

设是代数系统，+和∘是二元运算.如果满足以下条件:(1) 构成交换群(2) 构成半群(3) ∘运算关于+运算适合分配律则称是一个。

课文理解：Mice and rats are poisoned or trapped, while the gardener wages a never-ending struggle with ragweed, dandelions, slugs and root-rot. We have a modern arsenal of chemical weapons to fight off these invaders and we use them lavishly.Question: What do "chemical weapons" mean in this sentence?

A. guns
B. swords
C. arrows
D. insecticides