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

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

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Physics
Solids have definite shapes.This is because

The forces of cohesion of the molecules within are very much less in them
The forces of adhesion are less
The forces of cohesion are very large
The forces of adhesion are large

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