The word bird used to mean "young bird"; today it means "any kind of bird". This is called ______ in terms of semantic change.

A．It is nontoxic.
B．It has lots of vapour.
C．It doesn’t produce damaging fuels.
D．It costs less.

A．in gas form
B．putting it in liquid
C．preserving it in tanks
D．in the form of metal hydrides