When Is a Crossed Claisen Reaction Between Two Different Esters $\alpha$

When is a crossed Claisen reaction between two different esters synthetically useful?

A) When only one of the esters has $\alpha$ hydrogen atoms.
B) When both esters have $\alpha$ hydrogen atoms.
C) When only one of the esters has $\beta$ hydrogen atoms.
D) When both esters lack $\alpha$ hydrogen atoms.