  In this paper, we show a tight approximation guarantee for budget-feasible
mechanisms with an additive buyer. We propose a new simple randomized mechanism
with approximation ratio of $2$, improving the previous best known result of
$3$. Our bound is tight with respect to either the optimal offline benchmark,
or its fractional relaxation. We also present a simple deterministic mechanism
with the tight approximation guarantee of $3$ against the fractional optimum,
improving the best known result of $(2+ \sqrt{2})$ for the weaker integral
benchmark.
