Past exam of the mathematics course of the University of Cambridge / 2002 / iii / Paper 4 / 6 / i

No: there are at most three, including zero and the whole group. A two-generator ordered Abelian group is torsion-free and has rank of an abelian group at most two. A convex subgroup of an ordered group is pure: if $ng\in C$ with $n>0$, then $g$ lies between zero and $ng$ or their negatives, so $g\in C$. Therefore a proper inclusion $C<D$ of convex subgroups increases rational rank: an element of $D\setminus C$ cannot have a nonzero integer multiple in $C$. Convex subgroups form a chain, so a rank-two group permits at most two strict inclusions. This is the bound for convex subgroups of a finite-rank ordered Abelian group. It is sharp: lexicographically ordered group ${\mathbb{Z}}^2$ has exactly $0$, $\{0\}\times \mathbb{Z}$, and ${\mathbb{Z}}^2$.