When there are economies of scale,

A

MC $>$ AC$,$ so cost$-$output elasticity is greater than AC$.$

B

MC $<$ AC$,$ so cost$-$output elasticity is less than AC$.$

C

MC $<$ AC$,$ so cost$-$output elasticity is greater than $1.$

D

MC $<$ AC$,$ so cost$-$output elasticity is less than $1.$