Supposed you are given a directed graph $G$, a Hamiltonian path is a path that visits all the vertices (exactly once, since it is a path). Consider the problem of finding a Hamiltonian path in a DAG.

choose a or b whether:

a) One can compute a Hamiltonian path (or decide that one does not exist) in a DAG in polynomial time.

b) A polynomial algorithm that finds a Hamiltonian path in a DAG implies a polynomial time for SAT.