Some thermal containers have layers of materials to keep drinks cold for as long as possible. Which design would help keep ice frozen for the longest period?​

A. Inner core of steel, no air gap, outer layer of plastic

B. Inner core of steel, air gap, outler layer of glass

C. Inner core of steel, no air gap, outer layer of plastic

D. Inner core of glass, air gap, outer layer of glass


I know A. and C. are the same, but those are my options.