Let A = {$2^n | n$ is a positive integer}. A relation R on A is defined by $a^Rb$ $\iff$ a is a divisor of b.
Then the set A with respect to R is_________.
(A) a poset but not a lattice
(B) a lattice but not a distributive lattice
(C) a distributive lattice but not bounded lattice
(D) not a poset.