Earth’s mantle is
a. a layer of molten metal.
b. a layer of hot rock.
c. a dense ball of solid metal.
d. a layer of rock that forms earth’s outer skin.

d. new methods of assembly are introduced.

explanation:

this allows for the goods to be made on a larger scale. not faster because now it is shown to be made better, hence the greater productivity in the question. it is calling for something new to be introduced.