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Let 
$L$ be a finite distributive lattice. Let 
$\text{Su}{{\text{b}}_{0}}(L)$ be the lattice
$$\{S\,|\,S\,\text{is}\,\text{a sublattice of }L\}\cup \{\phi \}$$
and let 
${{\ell }_{*}}[\text{Su}{{\text{b}}_{0}}(L)]$ be the length of the shortest maximal chain in $\text{Su}{{\text{b}}_{0}}(L)$. It is proved that if 
$K$ and $L$ are non-trivial finite distributive lattices, then
$${{\ell }_{*}}[\text{Su}{{\text{b}}_{0}}(K\times L)]={{\ell }_{*}}[\text{Su}{{\text{b}}_{0}}(K)]+{{\ell }_{*}}[\text{Su}{{\text{b}}_{0}}(L)]$$
A conjecture from the 1984 Banff Conference on Graphs and Order is thus proved.
