  M.Aleknovich et al. have recently proposed a model of algorithms, called BT
model, which generalizes both the priority model of Borodin, Nielson and
Rackoff, as well as a simple dynamic programming model by Woeginger. BT model
can be further divided into three kinds of fixed, adaptive and fully adaptive
ones. They have proved exponential time lower bounds of exact and approximation
algorithms under adaptive BT model for Knapsack problem. Their exact lower
bound is $\Omega(2^{0.5n}/\sqrt{n})$, in this paper, we slightly improve the
exact lower bound to about $\Omega(2^{0.69n}/\sqrt{n})$, by the same technique,
with related parameters optimized.
