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【单选题】
目前普遍认为，质子和中子都是由被称为u夸克和d夸克的两类夸克组成。u夸克的带电量为2e/3，d夸克的带电量为-e/3，e为基本电荷。下表中关于质子和中子组成的两类夸克数目论断可能正确的是 [ ] 质子 中子 u夸克 d夸克 u夸克 d夸克A.
1 2 1 2
B.
2 1 1 2
C.
1 2 2 1
D.
2 1 1 1
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【判断题】In an open trading system, a country will import those commodities that it produces at relatively low cost while exporting commodities that can be produced at relatively high cost.
A.
正确
B.
错误
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【单选题】A service that consumes a relatively low level of resources but is reported to have a relatively high cost may indicate:
A.
Product cost cross-subsidization
B.
Product marketing
C.
Product overcosting
D.
Product undercosting
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【单选题】One of the reasons for the relatively high price of many drugs is the huge cost of ____and development.
A.
experiments
B.
research
C.
trials
【简答题】In electronicpower supplies there is often a need to isolate the output from the input andreduce the weight and cost of the unit. In other applications, such as inaircraft, there is a strong incentive...
【简答题】Pipelines offer the shipper an extremely high level of ______________ at a relatively low cost。
【简答题】在铁碳合金中一共有三个相,即铁素体、( )和渗碳体。
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