Physics
A nuclear reactor is said to have become 'critical' when

It starts emitting dangerous radioactive radiations
It is shut down to avoid explosion
It stops due to malfunctioning
It is ready to produce controlled energy

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Physics
The nature of fuse wire is

High resistance and high melting point.
High resistance and low melting point.
Low resistance and high melting point.
Low resistance and low melting point.

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