单项选择题

Which of the following did NOT cause so much trouble for the making of an atomic bomb.'

A．Extraction of uranium.
B．Sustentation of a reaction.
C．Conversion from uranium ore to uranium metal.
D．Separation of U-235 and U-238.
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Education, reserved to the states or the people in file U. S., is a ______ of the state, not the federal government.
A．condition
B．performance
C．requirement
D．function
单项选择题
Which of the following does NOT help to separate the useful U-235 and nearly useless U~2387
A．The principle of gaseous diffusion.
B．A process involving magnetic separation of the two isotopes.
C．A process nearly identical in the chemical makeup.
D．A gas centrifug
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