Indian, Pacific, Atlantic, Arctic Oceans are the 4
 
        
             
        
        
        
Answer:
Explanation:
Thuỷ sản nước ta thường được nuôi tại " ao hồ "
 
        
             
        
        
        
Answer: In Medieval Europe, knights pledged loyalty to vassals, who in turn pledged theirs to powerful lords, and serfs labored in the fields.
Explanation:
A rigid hierarchy refers to one where people who are born into a certain position in life, find it quite hard to move from it or rather one where there are different social classes and people had to be classified in one class for the system to work. 
The system during medieval times was much like this. Social classes existed that determined a person's worth in society such as the knights, vassals, Lords and Royals. There was also the serfs who were considered the lowest in the hierarchy. 
 
        
             
        
        
        
Answer:
<u>geologic unit</u>
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When reading a geologic map, rock units are labeled with letter codes. The first upper-case letter indicates <u>age of the geologic unit</u>, while the following lower case letters represent <u>geologic unit's name or the type of rock of which it is comprised.</u>
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Explanation:
Geologist have divided the history of the earth into 4 main periods of time; the Eons, the Eras the Periods and the Epoch. On a geological map, an initial capital letter is usually followed by one or more lower case letters while the lower case letters indicate either the name of the unit, if it has one, or the type of rock, if the unit has no name.
 
        
             
        
        
        
Answer:
The European Union is a political and economic union of 27 member states that are located primarily in Europe. Its members have a combined area of 4,233,255.3 km² and an estimated total population of about 447 million. The EU has developed an internal single market through a standardized system of laws that apply in all member states in those matters, and only those matters, where members have agreed to act as one. EU policies aim to ensure the free movement of people, goods, services and capital within the internal market; enact legislation in justice and home affairs; and maintain common policies on trade, agriculture, fisheries and regional development. Passport controls have been abolished for travel within the Schengen Area. A monetary union was established in 1999, coming into full force in 2002, and is composed of 19 EU member states which use the euro currency. The EU has often been described as a sui generis political entity.
Some of the rights afforded to EU citizens include the Following.
The right to vote and to stand as a candidate at elections to the European Parliament.
The right to vote and to stand as a candidate at municipal elections.
The right to good administration.
The right of access to documents.
The right to petition.
Freedom of movement and of residence.
Diplomatic and consular protection.