  The monotonicity method for the inverse acoustic scattering problem is to
understand the inclusion relation between an unknown object and artificial one
by comparing the far field operator with artificial operator. This paper
introduces the development of this method to the inverse crack scattering
problem. Our aim is to give the following two indicators: One (Theorem 1.1) is
to determine whether an artificial small arc is contained in the unknown arc.
The other one (Theorem 1.2) is whether an artificial large domain contain the
unknown one. Finally, numerical examples based on Theorem 1.1 are given.
