单项选择题

The solution, if that is the case, is to install a pressure-reducing（）on the water system.

A．shaft
B．shelf
C．axis
D．valve

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单项选择题
I don't like playing piano, for I felt （） to do what I was unwilling to do myself.

A．hindered
B．obligated
C．constrained
D．conspired
单项选择题
If a price or amount （）, it keeps changing and becoming higher and lower.

A．tumbles
B．tilts
C．fluctuates
D．flaps
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