Masons who assume that lime mortar, once cured, stays cured are often surprised to learn that years of rainwater seeping through cracked render can gradually leach out the binding calcite until the joints crumble, so that the hardening on which every wall depends proves, over time, quite _____.

Blank (i)
A. inexorable
B. ephemeral
C. reversible
D. superficial
E. imperceptible