单项选择题

The best way ______ the principle is to see how it actually works in practice.

A．understood
B．understand
C．understanding
D．to understand
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单项选择题
I felt somewhat disappointed and was about to leave ______ something occurred which attracted my attention.
A．unless
B．until
C．when
D．while
单项选择题
Only by shouting at tile top of his voice ______.
A．was he able to make himself hear
B．he was able to make himself hear
C．he was able to make himself heard
D．was he able to make himself heard
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