单项选择题

________ did Arthur realize that there was danger.

A．Upon entering the store
B．When he entered the store
C．After he had entered the store
D．Only after entering the store
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The mixture of elements in the air can be thought ________ a solution of gases.
A．of as
B．of
C．as
D．of to
单项选择题
The program has helped Chinese and American scholars ________ friendship and learn about each other's culture.
A．cement
B．stiffen
C．straighten
D．establish
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