Suppose $\langle a^{d_{1}} \rangle \leq \langle a^{d_{2}} \rangle$. Then $$a^{d_{1}} = (a^{d_{2}})^{m} \textrm{, for some } m \in \mathbb{Z}$$ Thus $$ a^{d_{1}-md_{2}} = e = \textrm{ identity.}$$