Let $S$ be a scheme and let $$0 \to A \to B \to C \to 0$$ be an exact sequence of abelian sheaves on $(\mathrm{Sch}/S)_\text{fppf}$. Assume that $A$ and $C$ are representable by flat algebraic spaces. Why is $B$ representable by an algebraic space?

I've seen this statement several times but never seen a proof. I also wonder whether this result has an analogue for the étale topology.