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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。

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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

Incityapartmentsanddwellings,thepresenceofcockroaches,fleas,ants,mosquitoesorhousefliesisguaranteedtoelicitthesprayingofinsecticides.Miceandratsarepoisonedortrapped,whilethegardenerwagesanever-endingstrugglewithragweed,dandelions,slugsandroot-rot.Wehaveamodernarsenalofchemicalweaponstofightofftheseinvadersandweusethemlavishly.Question:Which of the following is such an "invader"?

A. gardeners
B. insecticides
C. slugs
D. dwellings

The urban message kids get runs completely counter to what they are born with, a natural interest in other life forms.Question: For "urban message " , it could refer to the following answers except ___________.

A. Human beings are a part of nature.
B. Nature is dirty.
C. Nature is dangerous
D. Nature is our enemy.

The author of Hidden Lessons thinks that we have been deeply embedded in and dependent on nature since the history of humankind.The statement is___________.

A. True
B. False
C. Not mentioned