Explain, in terms of structure, why some polymers are thermosetting.

A thermosetting polymer, resin, or plastic, often called a thermoset, is a polymer that is irreversibly hardened by curing from a soft solid or viscous liquid prepolymer or resin. Curing is induced by heat or suitable radiation and may be promoted by high pressure, or mixing with a catalyst.

Explanation:

c. because step up transformer steps up the input voltage i.e increases the input voltage  

the number of turns are greater on the secondary side of step up transformer  

transformer does not handle dc current

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)