单项选择题

What is the main purpose of the passage

A．To outline contrasting types of economic systems.
B．To explain the science of economics.
C．To argue for the superiority of one economic system.
D．To compare barter and money-exchange markets.

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单项选择题

An addict cannot continue his education or work at a job, because of ______.

A．his concentration on getting drugs
B．his personality
C．his emotional problems
D．his shortage of money

单项选择题

According to the author, how can the addicts get the money for drugs

A．By borrowing from their families.
B．By hard working.
C．In an illegal way.
D．The author doesn't mention it.