单项选择题
Economies, several courses of which I have taken thus far, ______ to be difficult but useful for almost all students.
A．prove
B．proves
C．have been proved
D．are proved
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单项选择题
This book will show you ______ can be used in other contexts.
A．how you have observed
B．how what you have observed
C．that you have observed
D．how that you have observed
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单项选择题
All those youngesters will become ______ in ten years.
A．growns-ups
B．growns-up
C．grown-up
D．grown-ups
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