B.)C=C is the right answer.
because h<span>aving more double bonds than single bonds decreases the melting point and, hence, the fluidity of the substance this is why butter is a solid but canola oil is a liquid at room temperature.</span>
I believe this is what you're looking for.
A) iron melting is the only example of physical change, because it's just a change of state (from solid to liquid).
The others, rusting, combustion, and decay are all chemical changes, because it will result in changing the chemical bonds between the atoms.